I have a Canon D10 which is obsolete now but replaced by a D20 (which is smaller than the D10 and does HD video which are both nice things to have). What attracted me to the D10 was that unlike most underwater cameras which are good to 10' of depth, these are good to 30' (or 10m). It is essentially so good of a camera that it is my everyday camera (not just for when I go swimming or whatever). I'd say the D20 is totally worth a look.
The GoPros are great cameras but they are really meant to be worn, or mounted, to capture the "experience" of whatever it is you're doing. If you want to set up a shot then you need the backpack with a LCD screen and so on and the interface is really kind of clumsy (it's not really intended as an interactive camera). Don't get me wrong, I love the GoPros and what they have done to personal photography/video capturing of activities and so on but if you want a "look through the viewfinder/viewscreen and compose a photo" type camera they might not be the best tool for that.
They do have the wireless backpacks and iPhone/Android apps now mind you so that's pretty cool. If the intention is just to take tank shots underwater that might in fact be the way to go. Setup the camera on a pole or whatever and use your phone to see what it sees rather than craning your neck or using guesswork.
