If you want to do one of the snorkel trips I would book it in advance. I think you can cancel within 24 hrs of the trip if you decide to. The MiDushi was pretty booked in April and I didn't get my first choice of dates. You are going to have a blast!!
IMHO...depalm boat is far and away the best boat...snorkel or sunset sail with them is the BEST.See the boat on my pics in the gallery. Another stated april was pretty booked, yes, and the reason for that was it was school vacations in all of New England for 3 weeks of that month. You can certainly not expect that now, book now, or when you get there. The palm pleasure holds 100 people, and is quite large. You would never know that there are that many people on board while on the ship. cindyo
Depending on how adventurous you are a quad ride to the Natural Pool is a blast. I am not sure if any of the jeep tours still go, but it is something to look into. It is one of my favorite stops every year.
However, just beng there is enough to fall in love with the island, so enjoy and have fun !!!