Although I cannot speak for St. Martin, often the Caribb will have daily showers ... lasting for a few minutes to a wee bit longer .... many times I either will go into the water or sit with my clear poncho on as the sun, at times, does continue to shine! A heavier shower will send me to shelter (usually under a nice thick SeaGrape tree) to "wait it out." Many times, it has rained on part of an island but missed another part! And I have often found that weatherpeople can only guess what will happen .... Unless you know of some island that is protected by a domed roof so that no liquid sunshine can come down, weather is unpredictable ... You listen to your local TV stations, no? They are usually wrong!!!! ... Whether or not you cancel, (Far be it for me to say what YOU should do) based on a guess or "past history" of weather, is up to you and I do hope that others can give you additional positive reinforment and feedback specific to St. Martin!