I love snorkelling! My husband and I will doing some of that in Tulum, Mexico in October for our one year anniversary.
I've snorkelled in Mexico a couple of times now. There's an Eco park called Xel-ha that's pretty nice to go to. There you get the mix of salt and freshwater... So a real variety of aqua life. My second time started in Tulum with the sea turtles... Amazing. This year I'm going with the same tour company as I did in Tulum.