Whatever the type of case and camera you have, sand is your enemy. In Laguna Beach we love beach dives, that's what we do. Beaches here are like anywhere else in the world full of sand. And believe me this thing is nasty. Sand could cause havoc on your gear. If not removed systematically, sand will accumulate between the o-ring and the case edges thus reducing the sealing power of the o-ring. In most cases that will lead to water infiltration that if not corrected may result in case flooding. Case flooding means bye bye nice camera. Note that most camera warranties will consider flooding as missuse and will not cover the associated damages. See the section on flooding in this tutorial for tips on how to try to salvage your camera in case it happens to you.
The best way to avoid sand accumulation is by rinsing your camera case after each day of diving. Warm water will do best as it will also disolve accumulated salt that may find itself around sand grains thus helping in releasing them. With the case still wet from rinsing, remove the o-ring and use a dry Q-tip and rub it around the edges of the case where the o-ring sits. Don't do that when the case is dry as the Q-tip may tear out and coton leftovers between the case and the o-ring are almost as bad as sand grains. You can also use a toothpick to remove sand grains. Just don't use a used one. You also need to rinse the o-ring itself. Use papertowel on the wet o-ring to removed stuck sand and let it dry for a while before greasing it. See the next section about o-ring maintenance for details.
Remember to check for sand in every corner after each day of diving and you should be OK!

Unless you dive in a swimming pool and shoot swimmers, speedos or bikinis, you will need to rinse your camera case regularly even if you dive the Crystal River, the Cenotes or any other freshwater rivers or lakes. I would recommend rinsing the outside of the case every day of diving before taking the camera out and rinsing the whole thing (without the camera inside!) at least every three days of diving. Before removing the camera from the case, make sure that it is sufficiently dry. Water drops will probably not damage the camera but you want to avoid putting any water inside the case as it is a secret recipe for fogging. Let the case sit for a while in clean fresh water, preferably warm after your dives and then put it in a towel for at least 15 minutes to dry off before taking the camera out. Dive boats most of the time provide a bucket specially for camera gears. Do not rinse your mask in those as it will piss off all the other camera owners!

The o-ring of your case is what's make your case watertight. If the o-ring gets damaged or if too much crap accumulates between the o-ring and the case edges, your case will flood. The best way to avoid this is to regularly extract the o-ring from the case, rinse it toroughly (see section about sand above), let it dry off completely and then rub it with silicon grease. See the pictures below for details. Note that special very simple tools exist to remove o-rings (see picture). You want to avoid toothpicks or screwdrivers as you may damage the o-ring in the process. If you do not have a tool, simply squeeze the o-ring between your thumb and index finger on one of the corners of the case and it will slide out. Once it does, pick it up with your other hand.
Certain camera cases come with a flat seal, usually larger than the equivalent o-ring. Flat seals do not require silicon grease and it's even risky to grease them as it will make their surface uneven thus reducing their sealing properties. So if you have one of these, just rinse your case regularly.
Some case also have o-rings for buttons and switches. If you can easily remove and install these o-rings too you should also maintain them regularly, not as often as the main o-ring though. Probably twice or three times a year is enough.

Rinsing is an important part of the day-to-day maintenance of your case. As sand and salt may accumulate everywhere, I would recommend you immerse your case totally in warm water for a while and exercise each of the controls at least two or three times. You want to avoid the shutter button being stuck while diving don't you?
Avoid thermal shocks. If you've been diving the Vancouver Island (in a dry suit!), don't put your case in boiling water right away. Although plastic does not dilate easily thermal shocks are not good for cases. And, no, don't put your case in boiling water, that was just to illustrate the point...

Fogging may occur if warm and humid air is trapped inside the case. If this happens, the only way to get rid of it is to cool down the case which implies turning off the camera. If you're still underwater and want to shoot, you're screwed. Note that fogging is really nasty as it will happen underwater and disapear when you're close to the surface as the temperature inside and outside the case equilibrate. The best way to avoid this problem is to avoid fogging whatsoever. Before your day of diving, make sure you close your camera case in a cool and dry place. If you're traveling in tropical places, that means you should take the upgraded room with the A/C. If you're on a liveaboard, make sure their staterooms have air conditionning. Never open your case during the day as chances are you will be close to the water on a rocking boat and even if you take care of not letting water inside the case, the air is pretty humid there... If you're beachdiving in Laguna, you should be OK and fogging shouldn't be your problem unless you let water get inside the case. When the camera is on and especially if you use the flash a lot, it will heat. Since it's pretty much enclosed it will heat the air trapped in the case pretty rapidly. Trapped droplets of water due to their small size may heat also and increase the relative humidity of the air. That's a perfect combination for fogging!
Silica gel is conversly always proposed as a fogging prevention. Although there is some value in the little pouches that are invariably delivered with your brand new camera gear, there are some limits to the extent they actually work for our underwater needs. Silica gels are designed to absorb a relatively limited amount of humidity. Once they reach that limit, they need to be heated up to 125°C (or 260°F) for about 5 minutes to recover their potential. Unless you're travelling with your own personal oven, you're kind of outr of luck... In two dives with a few droplets of water in your case, the silica will be saturated and stop being efficient. Too bad. It also does not work at all in tropical destination when the air is so humid that the silica gets saturated by the air itslef! So to sum this up, you can put one little pouch in your case, it won't hurt, but don't expect miracles...

Easy answer: you're screwed. You'd better have a backup camera because you can kiss this one goodbye... But, if you're very very lucky you may be able to salvage the camera. First you need to remember if the camera was on when it flooded. If not, then your chances of saving your investment just increased. If it was on, well, you're kind of really screwed but you can carry on the salvaging process anyway (you don't have anything to loose, but don't expect much). Note that most modern cameras just like your TV are actually in stand by mode when they're off. That means that they are draining power from the battery, that also means that the associated electronics may have been fried during the flooding. Too bad. Anyway, the first thing to do after the flood is to extract the camera, remove the batteries and immerse the camera in warm freshwater. Yes I know that seems tough. If you're at home, distilled water will even be better, you can buy it from your local grocery store. If you're in Irvine, CA, be aware that the nasty poop water that come out of our sinks may cause more damage to your camera than salt water. Just kidding. Sort of. Anyway, once it's been freshwatered, dry off the camera. Use a hair dryer as you don't want to leave water inside the camera that may cause corrosion. If you can, use a toothbrush on the electronic circuitery to remove salt accumulation. However be aware that most electronic components used in cameras are surface mounted (even BGA) and that the efficiency of the process is not really guaranteed so you're probably better skipping that phase. Let the camera finish to dry for a while. Put back the batteries, sacrify a virgin or two and power it on. If it worked let me know as it never did for me...

If there is one thing to remember about the camera is to change the batteries after every day of diving. The best thing to invest on if your camera takes AA batteries is rechargeables and a couple of chargers. That will save you a lot of money. You can still keep regular AA batteries (alkalines work best) in case you forgot or were unable to charge your rechargeables. Redundancy is always good, believe me. Make sure your chargers take 110V and 240V and tolerate 50Hz and 60Hz. The smell of a burning charger is pretty nasty, believe me for that too...

Be aware that rechargeable batteries are not all made equal. You want to invest in high potency batteries. This is given by the number of mAh (milli-ampere-hours) that the battery is advertised for. The higher the better. 2,500 mAh is a good choice for an underwater camera. Also you need to be aware that the camera was designed for regular 1.5V AA batteries. Rechargeable are rated 1.2V most of the time. If your camera needs 4 of these, you end up feeding it 4.8V instead of 6V. That's a big difference and the Ohm law (remember high school?) does not play in your favor here and says that your batteries will get drained faster. If you can find 1.3V high potency rechargeables you will not regret your (small) investment. Note that the appropriate charger for this kind of battery is also needed although I have found out that the quick cheap ($12) foldable charger from Energizer does an excellent job. Quick chargers have the reputation of reducing battery life but I have not personnaly witnessed this.
If the camera takes a proprietary battery, make sure to buy an extra one so that you can rotate easily. Usually the one sold with the camera will not take you through more than two dives, so just like rechargeables AA , you should invest in an extra higher potentcy battery. An extra charger may come handy if you waste two batteries a day.

If you have an external flash, it'll need batteries too, so the same as above applies.

For the rest, nothing new for underwater, just keep your camera clean especially the lenses, don't forget to charge it with a data card before you jump in the water (it's been reported deadly for the camera to insert a CF or SD card underwater - if this happens to you, see the flooding section above). If you're travelling make sure to either bring a sufficient number of media cards or best, bring a laptop. The newest $300 netbooks are the top for us globe-trotters. Very light and ample storage for our pictures. Having a laptop available will also help you improve your technique as you can review your shots every day after the dives. If you don't travel with a laptop, I would recommend you bring some DVD-R or CD-R so that you can backup your photos on someone else's PC in case you run out of space on your media (believe me, you will, shooting is addictive!). Remember to either bring a cable to connect your camera to your PC and install the appropriate drivers on the PC if your camera is not seen as a regular mass-storage USB device or to bring a USB media reader ($10 max). Note that most modern laptops have media slots, just verify that they can read your media.