We took several snorkel trips, using a different outfit each time: all of the docks have operators, price signs posted, schedules, etc. (many of the schedules are the same, e.g., Hol Chan two times per day, a.m., p.m.), and the prices were all within about $5 US of each other (booking through the hotels may take a little more, but I suspect not much; the commission is probably paid by the tour operators). We just walked up and booked ourselves, very easy. However, I'd like to recommend White Sands; they are a bit north, but they came to get us at our dock (though many do this, some act put out to be bothered) and took two people, snorkel only, when others insisted on a 4 person minimum, and didn't charge us extra, as others would have. We had a great guide, Carlos, and he was not only friendlier than other tour guides we encountered, he was also very accommodating and professional. So, they were a little farther afield than the "townie" operators, but we liked them the best of the four tour groups we used.