We have been to Couples Negril several times. The nude beach is nice, there are a few people that walk by at the waters edge, and look, while trying to appear not to be looking. It's not a big deal.
You can take a snorkel trip with the resort, but it's not nude. There is a really nice hot tub on the nude beach you can watch the sun set from. I suppose you could wear snorkel gear while you do it but......yeah your right it wouldn't be the same.