we were there in august and hubby dove in a t-shirt and trunks. most folks prefer a skin. dont think you need more. two guys at our place got blisters from their fins, so bring some of those waterproof (nexcare) bandaids. they worked out well.
may i mention 2 things: he dove with dolphins at turneffe and would highly recommend it, and skip the blue hole, says it is so deep you dont really get much bottom time. he also saw sea turtles i think at hol chan. enjoy.