What would you recommend for taking photos under water? A camera dedicated to it or a normal camera with a waterproof case?
The Pentax Optio W20 and 30 (http://www.steves-digicams.com/2007_reviews/optio_w30.html) and the Olympus Stylus 770 SW (http://www.steves-digicams.com/2007_reviews/stylus770.html) are the only waterproof cameras I could find. The Olympus is a lot more expensive than the Pentax but it is also more solid (it can survive falls up to five feet, still work at -18 C etc).
I looked at waterproof cases and they can handle -40 metres underwater which is a lot but they are as expensive as the dedicated waterproof cameras.

I know nothing about waterproof cameras,but why do you think for a waterproof case for one of your cameras?
http://www.usa.canon.com/consumer/controller?act=StandardDisplayAct&fcategoryid=113&keycode=underwater_photo

I would like to take photos of fishes in the sea or in the river. A photo/movie of a dog swimming taken underwater can be fun too.