Like the above post indicated having a back-up plan would be key. One hour isn't a whole lot of time should the flight crew be delayed, a mechanical issue arise with the plane, etc. Been there and done that. I'm not sure any other airlines fly to SXM from Newark each day if your particular flight got delayed. But, you might want to check in advance with United to see what their policy is. I'm quite sure that both Delta and Jet Blue have St. Martin flights from JFK every day, so if those two had later departure times you could possibly switch airports if time allowed (and if United would go along with this). It's also possible (though unlikely) that United would delay their SXM flight waiting for delayed passengers, but you could always inquire about this. I have seen that happen with some other carriers.

One other common sense item is be sure to pack one full day of clothing, swimsuits, etc. in your carry-on bag in case you make the flight, but your checked luggage gets delayed and arrives the next day. Hope this helps!