Dunedin Causeway and Caladesi Island. It's been hard, so things may have changed. If the tourist numbers increased or a hurricane caused problems there may be less, but they were great a decade+ ago.
I've never even heard of those places, so I googled them and saw they are just north of Clearwater. Oh, my wife lived in Clearwater many years ago, so I asked her. She's heard of them, but she wasn't into snorkeling back then. I wonder if anybody else has ever snorkeled there.