I lived in a house in Sandy Point on St. Kitts for five weeks and worked at the Brimstone Hill Fortress doing archaeology. I have to say that is the best way to enjoy the island. The locals are friendly and the black sand beaches are beautiful. The buses are exciting to ride, and I definitely recommend going to the Black Rocks & seeing petroglyphs.
Taste the johnny cakes (which differ at each restaurant) and try the guineps (mamoncillos), too. They are sour, sort of like grapes with a pulp and shell covered seed. When we were returning from Nevis by ferry, a nice man gave me and my friend each a bunch of guineps. One of the workers at Brimstone Hill would also bring us kids several bunches throughout our work period.
Like the previous poster said, seaweed can cover some of the beaches, but it really depends on where you go and most commercial beaches will clear it up. (Turtle Beach is a wonderful place to swim and eat – tho expensive food. They even have – or had – a large pig named Wilbur that would walk around trying to eat your food, and monkeys sometimes play on the porch.)
If you’re an American girl, esp. blonde, take care – those boys will swim out to hit on you! Be aware that the locals hate the vet school students, because they don’t take an interest in local events. Otherwise, you’ll have an excellent time.
The picture is a view of Sint Eustatius, I believe, from Brimstone Hill.