Have you ever tried Australian Gold sunscreen with a 50 spf rating? I found this sunscreen to be a bit different than others I have tried, and it sort of has a bronzing agent as well which helps minimize the sun's damage. I used this with a 50 spf rating the first few days on a trip to Playa del Carmen last year and nothing got thru (and I mean nothing). Switched to a 30 spf sunscreen the balance of the week so people actually knew I went to Mexico, and I got an awesome tan that really lasted. Might want to give this a try.

One other small tip I learned a long time ago is to apply sunscreen everywhere in the morning (in the buff) before you get dressed. The St. Martin sun has this ability to find small openings and roast you really good even if your just walking around and not on the beach. Oh, and don't forget your lips! Burned them really bad on a past trip and paid the price when I ate barbecued ribs later that night. A lip balm with sunscreen is a wise move. Hope this helps!