The islands are warm and the water temps are body perfect. But the islands are warm, especially for a roll of film sitting in the sun on a boat deck or in a bag on the beach. So I highly recommend a collapsible cooler bag. The size of a six pack of sodas, or big enough for all your film and a blue ice. If your room has a fridge with a freezer you can put the blue ice pack in every night, ready for use the next day. 'I will have some frozen blue ices ready every morning'. I also travel with a small white towel to put over the bag as well. Hotels frown on guests borrowing towels. Also the thing about beaches is they are quite sandy and that sand seems to find its way into every place you don't want it! You might want one of those very large (sport) zip lock bags for keeping splashing water at bay. Check with your sporting goods store and tell them how much stuff you want to take out on the boat. They can guide you to the right size etc.
CHECK LIST
collapsible cooler bag
large water proof sport bag
small blue ice (for travels without Chel)
small light colored towel