Boca Catalina Beach
Mangel Halto for Snorkeling
We've also used Roberto's (twice) and enjoyed it very much. Aruba Bob is excellent as well, but they are very different experiences. Roberto's is a smaller tri-hull sailboat with usually 20 - 30 snorkelers. Aruba Bob is a personal tour with great guides.
I took the Roberto's snokeling trip last November. There trip was very good. They have a tent just north of the Holiday Inn and you can book your trip there. Roberto's daughter was maning the tent and the son does the tour. They leave from that location to the dock nearby. Aruba Bob's is a great experience also. When out with them in 2011. Would recommend both.