We were in Cozumel in the end of April in 2000 and it was 98degrees in the morning........ i doubt if it gets too cool at night. havent been to key west tho. The weather is usually in the 75 to 80's in the western Carribean.... enjoy........ Char
Depending on what part of the country you are from, you may notice the higher humidity. It should be nice and warm in April, and don't forget the sun is very strong, so pack that sunscreen. In the evenings when the sun sets, you may get chilly, but mostly in the dinning rooms for dinner, it can feel chilly, so be sure to pack a light sweater for just in case.