I just returned from San Pedro following a 10 day dive trips. Water temperature was 81F throughout and I was fine only wearing a full length 3mm wetsuit (I also brought with me a Bare Chicken vest just in case). Having said that, some folks in our groups wore 5mm and they were also happy with their choice. BTW, some divers even dove with neoprene beanies over their heads.
In addition to the boat ride, climatic conditions and boat type will also dictate how comfy one will be. Our first two days of diving were ''blessed'' with windy conditions (wind from the north), substantial swell and waves, cloudy and scattered showers. Add to that the occasional sea spray and a full bladder for the return trip. Even those wearing 5mm found it cool topside after a dive. Due to the proximity of the sites and also on the sea conditions outside the barrier reef, it seems that most dive operators prefer returning to their dock (transiting inside the reef where conditions were much smoother) during the surface interval to exchange tanks, empty bladders, etc rather than having their divers be tossed around for an hour or so.
On the other hand, we were blessed with great conditions for the last fours days of diving with moderate winds (it seems there is always wind in Ambergris Caye), sunny and warm and only choppy sea conditions which made the transit to and from the dive sites truely enjoyable.
