It does rain in St. Barts, but it doesn't last long.  There have been times that it rains on the beach and people in the water did not botrher to get out.  October through March is the high season. Less humid and in  the evenings a wind breaker is good especially if you are near the water. It is more expensive than other islands and many, many tourist.  The Harbour is filled with yachts and there is excitement in the air.

Some prefer the off season as there are not as many tourists and just as much fun. It is great for the budget minded. July and August are the quietest months. Casual dress is the best way to go. St. Barts is a very relaxed island.