[Arubalisa]

For serious boat snorkeling, Tranquilo http://www.tranquiloaruba.com/. It is a long day though with the sailing included. All in all a nice relaxing day. I wrote about our experiencehere and have photos here.

I do not know when you will be on the island, but Tranquilo will be off on vacation racing their sailboat Oct. 1 - 12.

For light hearted fun with some snorkeling, lunch and dancing, Jolly Pirates. Specificially their "Sail, Snorkel, Swim & Swing"
http://www.jolly-pirates.com/web/Tours.php

I wrote about our experience here and have photos here.

Also see additional info at http://www.aruba.com/forum/tags/snorkel%20cruise.html