In the South Atlantic, the snorkeling at Ascension Island is very popular. This is a dry volcanic island with few sandy beaches. While snorkeling off Ascension Island, you may encounter hawksbill turtles and green sea turtles (December to June).
There are a range of shore-accessed and boat-accessed dive sites. The most popular shore-accessed snorkeling site (and popular beach) is English Bay Beach located northeast of Georgetown on the north coastline. Alternately, another good shore-accessed snorkelign site is Comfortless Cove, located just north of Georgetown.
The challenging boat-accessed dive sites include South East Bay, Boatswain Bird Island (known for reef sharks and manta rays), Boatswain Bird Rock, Porpoise Point, and Lions Mane Reef. The shore-accessed snorkeling dive site include North East Point, Kettle Cove, Triangles, English Bay Beach, and Comfortless Cove.
There isn't much of a tourism industry on Ascension Island. You'll be pretty much on your own for gear and arranging to visit any of the snorkeling dive sites. There is an informal Ascension Divers Club that can provide some assistance -- it is located at North Point. If you're interested in the boat-accessed dives, it might be easier to find someone to take you out there on the weekends.
In order to visit Ascension Island, you will need to apply for an entry permit at least one month prior to arrival that requires proof of travel and medical insurance. The usual route to the island is the nine hour flight from Brize Norton, UK.
Overall, Ascension Island is the 1st most popular snorkel dive spot of all 2 snorkeling dives in Saint Helena - Ascension.