It is a popular and great reef for snorkeling and diving. You can rent the equipment for snorkeling and diving at the entrance. They have nice and experienced guides.
Avoid cruise ship days. It's completely crowded. If you can't avoid go early not later than 9 a.m. or in the afternoon. Sea is calmer in the morning. Of course best with sunlight. April and May higher chance for sea turtles.
The dry season last from December until April/May. Cruise ship season last until late March. Rainy season from June until November with the possibilities of hurricanes and flash floods. The driest month is March and the wettest is August.