Don't really k now about the snorkeling, but the scuba diving is world class. They have a lot of ppl who do diving there: Red Sail Sports, Don Foster's, and Eden Rock are all dive operators that I've been with. All of them were excellent, Red Sail tend sbe the most expensive though.
It's been a long time since I've been there, but Eden Rock was "the" shop and for a good area that hasn't been mentioned, go a bit south of town and you'll come to a small clearing called Smith Cove (or something similar) and you can go in from there. Saw a small shark there once, but it's a good, easy area to go in from and if you just want to not get into any current, it should work well for you. I've not been there since the late 80s, but it was about 10-15 mins by foot from Eden Rock.