It's been five years since we stayed at CN, so it may have changed, but here's my two cents:
Unless you bring your own snorkel gear, you need to go out on the actual snorkel excursion. You will be with a group of people (8 - 12) and I never saw any do it topless. But there's always a first for everything
There last February and didn't see anyplace that appeared good for snorkeling.Swimming area roped off with no structures to attract the fishies!Outside the roped area is dangerous due traffic trying to rent you jet skiis and para sailing with an occasional fisherman going by.For snorkeling I'd do an excursion.Been on many snorkeling trips and never saw a female topless snorkeling, however I was looking for fish!
The roped off area really has no structure to attract much to see, however, my wife and I both snorkel there for something to do while in the water. We have seen many star fish on the bottom as well as some bait fish. A couple of times we have seen ray's in really close and of course crabs from time to time. My suggestion would be to do the snorkel trips. As far as topless, I always am but I have never seen a woman snorkel topless except in Playboy and I am not certain where she was.