It's not the heat that scares me about south, it's the crowds. Even if we look at the photos in this thread, most of them look way too crowded for my liking.
I agree about the photos looking crowded. The Eastern Caribe is way crowded with very few places to escape from them, and most of those places are just a few less popular anchorages and not really escaping the crowds because you are right back in them as soon as you pull anchor.
However, the Bahamas have many places where you will be isolated (maybe one or possibly two other boats occassionally) for 100nm around. Jamaica gets few cruising boats and a ton to explore and do. Cayman Brac and Little Cayman are isolated with pristine diving and snorkeling. Going West are the Swan Islands where only one or two boats a year ever stop, then the Bay Islands of Honduras that have a few crowded popular anchorages, but many more where few ever go. You can find a lot of isolation in Belize outside of the few popular charter places. Heading South from there are Providencia, San Andreas, Albuquerque Cays, and Corn Islands, of which Providencia and San Andreas might have a half dozen cruising boats, while the others rarely see any. The San Blas Islands of Panama can be a bit crowded on the couple of far Western end islands, but the 300 islands to the East hardly ever see cruising boats. Even the Bocas del Torro Panama archipelago has two or three anchorages that are full of cruisers, while hundreds of other anchorages remain empty.
Go South, but go West for isolation, cultural variety, and incredible nature, not East.
Oh, and you should be scared of the heat...