If you are going with an organized tour, I would say the most spectacular cenote for snorkeling is Pet Cemetery. It may cost you a bit more, and it's a long drive from the highway, but it is truly spectacular. There is some gorgeous snorkeling at Grand, and also at NoHoch -- but NoHoch is tricky; you can get truly lost there, even on the surface. (Some cave diving friends of mine did some snorkeling there one day and scared the stuffing out of themselves!)