Good ideas all, and thanks for contributing.
Honestly, though, the idea was for them to unite post 1900. In spite of the difficulty, it's much more interesting to see how such a nation would deal with the language barriers and how those barriers would culturally break down over the years (via slang and hybrid tongues). I know this wouldn't occur fully by present day, but it would be interesting to hear a Jamaican English and Haitian French influenced Spanish develop.
The economy is another thing I was considering. They are all still mostly agrarian or semi-agrarian in the early to mid 20th Century. Would unification lead to greater industrialization?
Culturally and musically I'm also fascinated. There are as many different styles of music and dance (and subgenres therein) in the Caribbean as there are fish in the sea. A Caribbean musical umbrella genre gaining popularity by the sixties is sure to include mento, salsa, ska, proto-reggae, etc. and would certainly effect future genres of world influenced popular music.
And of course there are the political ramifications. Due to constant interventionist policies by both sides of the Cold War, many of the Caribbean nations are prone to either communist dictatorships or western puppetry (or the constant violent revolutionary in-between). Could unification lead to the confederation of island states finding progress in playing both sides in the Cold War (i.e. a soft dictatorship run under the auspices of democratic socialism? Perhaps accepting U.S. and Soviet arms and funding to clandestinely support both sides' goals in the region...) What could be the future effects?