There’s a couple of great places for snorkelling on the Isle of Portland, one either side of the causeway.
Chesil Beach side, in very calm weather only, you can swim off the eastern end of the beach near Quiddles and head off with your mask and snorkel south along the bottom of West Cliff.
On the other side, inside Portland Harbour, you can snorkel pretty much anywhere at anytime and in any conditions, I prefer going near the Castle which is only a couple of hundred yards from Kearney Cottage but there’s also an excellent spot across the other side of the harbour near a beach cafe called The Shack which at times has been known to do Champagne Cream Teas on the beach, quite an experience.
Under the water there’s a good selection of fish but the things to keep aneye out for are the crabs, ranging in size from 1″ to about 12″ monsters, if you’re lucky you might even witness a couple of the larger dominant ones fighting over food or territory, when they’re doing that keep your hands close to your body.