Remember it is the hurricane season until end of September-ish which is why there are not many cruises there during August!
RCI Carnival Celebrity & others cruise the Caribbean during August. Why don't you speak to a travel agent & let them do the work for you. There's a freephone number top right of this page.
I would forget about the Caribbean next August if I were you. I have just heard reports of the first really bad storms of this hurricane season heading towards the area, and we are only just into July! If you really want to cruise the Caribbean, try to stick to the traditional winter months, if the cruise has to be at that time of year, there are hundreds of lovely ones to choose from in Europe. You will have much more choice of cruises whichever of these options you choose compared with your original idea which will almost certainly have to be on an American Cruise line (filled with Americans). Happy cruising.