Just north of Nail Bay is Long Bay. Amazing shallow snorkeling along that beach. North of that is Mountain Point for more great coral. It's the only place I have seen an octopus.
Mahoe is very nice as are each of the other beaches mentioned. I would also suggest the north side of Prickley Pear, the Anguila cut and Eustacia reef. If you are land based, you can rent a dingy at the Leverick Bay Marina and hit the North Sound sites that way.
Mahoe and Savannah have great walk in snorkeling - the baths and devil's bay can be difficult if there is a swell or major current coming around the bottom of VG.
.
