Honestly, in all my years of traveling the Caribbean, I have never had an entire rainy day on any island. Sunandfun is right. If it does rain, it's a quick afternoon shower and it's over before you know it.
If you did have the unfortunate experience of a rainy day, I would say it would be a good opportunity to do some shopping. Keep an eye on the weather and if you see a rainy day ahead, use that day to get your souvenirs and whatnot.
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