Never had an issue with tap water in St. Martin or any of the other nearby islands. Now, Mexico is a much different story. Sounds like they're simply trying to get you to spend $6 on something that is typically free of charge. If the water was an issue, then it would extend beyond drinking tap water. For example, when you eat a salad they would wash the lettuce (hopefully) in tap water and that would still cause potential digestive problems. So would ice cubes, etc. I wouldn't worry too much about this when visiting SXM, but for Mexico (Cancun, Tulum, etc.) we take probiotics one month before and after our visits. We never have done this for St. Martin.