More stuff to add if I may.
Weight and balance
Get a camera that has a bottom weight system so that its not only neutrally buoyant but also stays upright. This way you do not end up fighting to position the camera especially if there is a current cause you'll probably be more focused in positioning yourself relative to the flow of water to get the shot.
Button Layout and orientation
This might sound silly as well but its also good to bring along your dive gloves and test the set with gloves. Gloves numb the feel of the camera and sort of screws up your mental orientation of your finger tip positions. Thus closely positioned buttons might pose a problem for ham fisted folks particularly those compact camera series.
Check to see if the commonly used buttons are in easy reach and ergonomical to your hands. Marine life will never pose for you and significant movements may scare them away.
Reduce risk of leaks
Get cameras with good battery life and memory cards with big memories, for you would not want to fiddle with the camera to change batteries or cards whilst on a trip.
Its best to take your time on a Thurs nite to carefully clean the case, ensure the rims are lint free, carefully grease the O ring (in a bag), apply it onto the casing, pop in the cam and carefully closing it checking to make sure the O ring sits well. Also do a gradual leak test in phases starting by running water, surface immersion then bottom of the pail immersion.
Casing's Integrity
Some earlier models (talking in general here) had weak spots that were prone to crack when impacted at a certain angle. But I trust that you rarely will have this problem today.
Take a close look at the O ring seat. If they have a demo unit with the O ring in place, gently rub a finger or 2 across the rim to see if the O ring jumps out. a fairly well recessed or seated O ring will not have this problem. This minimises the risk of O ring mis-seating that causes leaks. Also test the lock and catch system. Some casings have 2 and its always good to be comfortable and familar with them lest you resort to violence or use force cause you put yourself at risk of having the setup dropping.
You may also want to check out the range of accessories available like add on lens, strobes for future expansion. Sometimes compacts can take better pics simply cause they simple easy to handle. They just need better lighting or accessories. When you're comfortable with your cam and have mastered its settings I am sure you'll take great pics.
Security
As not to lose your investment ..... invest in a lanyard type device.
