Mike gave you some great suggestions for a cruise. The temps in the Caribbean don't vary a huge amount thru the year. It will be warm its summer after all. It will be the hottest on shore in town. There is always a breeze off the water on the ship. I would stay away from Bermuda in August, mainly because if an Atlantic coastal storm/hurricane comes around, there is nothing to protect Bermuda its just out there.
Royal Caribbean would be my first line choice - the Voyager or Freedom class of ships.