We've been to the Mexican Riviera several times as well. Very near Tulum is a little village or marina called Puerto Aventuras. This place is awesome. There are only three hotels there, and you can snorkel right outside the hotels. One in particular (I'll get back to you with the name) had FABULOUS snorkeling. And it was walk in and swim a short way to the reef. It was VERY convenient to everything. XCARET was only a short taxi ride away. As was Tulum, XELHA, etc. Being a marina, deep sea fishing boats were right there. There was also a large ferry that docked there that went to Cozumel daily. The marina has a type of board walk around a gorgeous lagoon, and the lagoon has dolphins in it. You can have lunch on the lagoon and watch the dolphins play. I can't say enough about the place. Even the shopping was good. As for Ambergris, the entire place was wonderful. So relaxed and has such simple beauty. The reefs were way better than the ones off Cozumel, and shark ray alley was an experience I will never forget. You can't go wrong either place. Have fun.
Debbie