The Westin is a little isolated, but upgrades Platinum well. The snorkeling right at the resort is pretty good. I would recommend renting a car, at least for a bit. The snorkeling at Cozumel is great, and there are some nice beaches there. I've stayed at the Westin twice, and I really like it. For points, it's the best place I've stayed in Mexico. If you wanted to mix it up a bit, the Occidental there (all inclusive) has a lovely beach, good snorkeling, a dive shop on premises that offers snorkel trips, good food, an adult pool, and it's a fairly small resort with not too many kids. I've stayed there twice and been very happy. Cozumel has great snorkeling, a lovely town, and isn't too touristy.