the 'wetbulb factor' applies whenever there is an unlimited water supply (always), air moving over the wetsuit, and the heat source is your body. it would also apply to a drysuit that is wet on the outside although the heat transfer is much slower and the effect would not be as pronounced.
a dry suit works by trapping air between the suit and your body. in the case of the drysuit, the insulation is the clothing you wear under the suit. i doubt you could float out enough ait to significantly reduce the insulation value of the clothing you wear under the suit (typically fleece or long underwear). they work very well, in fact, most likely too well for florida except when we get one of those really cold days.
my brother dives for noaa and used a drysuit for a few years when he was at woods hole in mass. he is now in florida and doesn't plan on using it until he is reassigned to somewhere cold again. he said that even his new suit sometimes leaks leaving his clothing under the suit wet at the neck and wrists.
the object is to keep the air in the suit from moving around thereby reducing the heat transfer due to convection. the problem with a dry suit is they do not breathe. any sweat will stay inside the suit and if it is not cold enough, you will become very uncomfortable.
if you are planning on staying on the boat and getting wet every so sften due to the boat flipping (like a few times a season), i would stick with a thin wetsuit and a spray suit over it.
a calculator can be found at
http://www.pipingnews.com/psychrom.htm situation 3 is the one to use. it gives a whole bunch of other data that is useful in HVAC design, but not too useful for this application. for those of you who do not know, I-P is inch pound units
by the way, i read the psychrometric chart incorrectly. the wetbulb is actually near 63 degrees at 75 drybulb and 50% rh. i have been using these things for over 10 years and still misread them every once in a while. if anybody wants to see one, let me know. i can scan one and post it so you can see what i'm talking about.
