If you're seriously looking for an underwater camera, Olympus will be your only choice.
True, you could get underwater casings for other brands, but that'll easily cost half a new camera.

for dslr camera i dunno la..total noob for slr but for the u/w camera, i got something to say and ask u..

for which extend u want ur picture to look like?
are u just a snorkeler? or a scuba diver?

if u are a scuba diver, the current olympus is not enough...
the current model of olympus which can withstand shock and water proof, does not have the manual setting and all u wil get from the camera is pictures which is blue and dull cause taking photo underwater is total different experience from the land. U have to take into consideration the water column and the lost of the colour when u reach even one metre of depth.

So, find those camera which have the manual setting which u can play around with the shutter speed and aperture.

i was once u, bought the olympus model which can be brought down to water more than 10metres but regretted it once i got the photos..

now i'm a happy G9 user... for land i still use my mju 770sw, the first olympus camera which can be brought down to the sea.