You sign up for the international plan which is $10 a day in most countries (like 140), the plan is better for Mexico and Canada but cant remember how.
You get like 140 MB of fast data and if go over that they charge another $10. But, you get unlimited calling and messaging as i recall. Check all that but i used it in Germany and Prague last Summer and worked well. Maybe went over once.
Download Google map for St martin to your phone then you can use GPS w/o data use.
If you wait until on WiFi at hotel or wherever to send pics and stuff the 140 mb will last you for quick search’s and such.

Make sure you add the international plan though, if not charged by the second and very expensive. If you don't turn on the phone on cellular/data you aren’t charged that 24 hours.