Yeah, a full 3mm will do most places. Use it more of a protection from coral (you really don't want to get near fire coral... trust me). Some times currents underwater will just slam you into stuff.

Besides you want to own your own wet suit... some people just like to pee underwater, and last thing you want is a rental that reaks of urine and salt water (yeah, its been washed, but still!).

If you make it down to cayman, give me a shout! I'll take ya to some local dive sites... show ya around.

Kris