Rosebud is right. It also depends on the amount of dives you are doing. I was there in December. The water temp was 71 degrees. We did two days of diving, 4 dives per day. On the first day, I just wore my 3mm and a beanie and by the last dive, even though shallow, I was getting a little cold. The next day I wore my 3mm, beanie with a 5mm core warmer and was comfortable all day. We are going back in July and even though I know the water temp will be much warmer, I will still bring along my core warmer just in case. Remember, even though the water temp is in the 80's your body still has to work to keep it at 98.6. If you are only doing 1 or 2 dives a day, I would just wear a 3mm or dive skin. I like the 3mm because it gives me more protection, but that's just me.
Have fun in Bahamas. It's a beautiful place to dive!
janet