Both of these cameras are great starter DSLRs. Nikon DSLRs tend to have better image quality than Canons at high ISO. Having said that, I suggest you go with whichever you can find a better deal on. Even consider dropping down from the T2i to the T3 to save a bit more money.
However, neither of these cameras are safe to shoot with underwater unless you're also going to buy a water-tight housing. And in many cases, the housing costs nearly as much as the camera itself (and sometimes quite a bit more).
