If you are interested in a snorkeling trip, it is easy to walk down Hidalgo and go up to a shop that offers that and sign up for a day later in your trip. However, if you would prefer to go ahead and set up something ahead of your trip, you can set it up with Julia at Julia@everythingisla.com.
Julia can also set up a sport fishing boat or a scuba trip.
Scuba is offered at all of the snorkeling tour guide shops along Hidalgo but if you want a trusted recommendation, you can book with Julia at Everything Isla, ask the hotel concierge or even ask a local scuba shop if they know of a good one to go to in Isla Mujeres.