It does rain in St. Barts, but it doesn't last long.  I have been on the beach and when it began to ran the folks in the water did not get out.  October through March is the high season. Less humid and in  the evenings  a wind breaker is good especially if your near the water. It is more expensive and many, many tourist.  The Harbour is filled with yatchs and there is excitement in the air.

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