We always bring our lycra suits, they are thin, but comfortable. 80 feet down gets cold after awhile. And when we use it for snorkeling, I don't get sunburned, and you are less likely to get any jellyfish stings.
On the fins question, YES I think you really need to have fins, you're legs will get mightly tired if you snorkel all the way out at Castaway and anywhere else. There is always the chance of waves and currrents and you will need the extra help that fins supply. Remember not to walk on the beach with them!