The Barmouth is an estuarine snorkel site located where the River Bann meets the Atlantic. Snorkellers access the water from the shore on the Castlerock side of the pier, where rocky topography defines the underwater environment. This site is suited for intermediate snorkellers due to the variable conditions of the river mouth. Water temperatures range from 9C in the winter months to 15C in late summer. Species recorded around this area include harbour and grey seals, catsharks, skates, and various invertebrates such as common lobsters, rock crabs, and tube worms. Compass jellyfish and giant jellyfish are also occasionally present during the summer months.

The Bann Estuary, located at the eastern end of Castlerock Beach, offers a unique dive experience where the River Bann meets the Atlantic. The site features long stone training walls and an artificial reef constructed from pier boulders. Divers can expect depths ranging from 3 to 7 metres, with an average of 5 metres. Access is via the shore, though conditions include moderate currents and visibility between 1 and 5 metres. Due to the presence of boat traffic, cold water, and potential rip currents, this site is best suited for intermediate divers. Facilities available on-site include parking, toilets, showers, a rinse station, and a local dive shop offering guided tours.

Castlerock Pier and Rocks is a shore-entry snorkelling site located near the mouth of the River Bann in Strangford Lough. The topography consists of rocky areas surrounding the pier structure. Conditions at this site are variable, as visibility can be influenced by discharge from the nearby river. Water temperatures range from 9C in the winter months to 15C during the peak of summer. This site is suitable for beginners, though it is rated 2/5 for snorkelling activity. Divers and snorkellers should be aware of the influence of the river environment on local water conditions.

Castlerock Basalt Cliffs (West End) features basaltic rock formations and deep rock pools where the sandy beach transitions into a rocky reef. This shore-entry site is suitable for beginners, with depths ranging from 1 to 5 metres and an average of 2 metres. Snorkellers should be aware of icy water temperatures between 9C and 15C, as well as potential hazards including surf and strong currents. Species recorded around this area include catsharks, skates, various crab species, and jellyfish such as the compass and giant jellyfish. Parking is available nearby for easy access to the water.

Portnahapple is a sheltered shore dive located in Strangford Lough, offering a mix of reef, kelp forest, and artificial reef habitats. Divers can expect depths between 8 and 10 metres, with an average of 7 metres. The site is suitable for beginners and features mild currents. Marine life commonly observed includes catsharks, European green crabs, and snails. Less frequently, divers may encounter compass jellyfish, giant jellyfish, skates, crabs, and tube worms. Visibility ranges from 5 to 15 metres, and the site is accessible for night diving. Facilities at the shore include parking and toilets. Typical dives last approximately 45 minutes.

Foul Grounds is a drift-snorkeling site located north-northwest of the harbour in Strangford Lough. The seabed consists of a mix of coarse sand, cobbles, and boulders. Snorkellers may encounter a variety of marine life, including snails, which are very common. Less frequently, observers record catsharks, rock crabs, tube worms, and European green crabs. Rare sightings include skates, giant jellyfish, various crabs, rockpool prawns, and shannies. Due to the nature of the drift, this site is suited for intermediate snorkellers. Access is by boat, and water temperatures fluctuate between 8C and 15C throughout the year.

Portvantage Cove is a rugged, rocky reef located in Strangford Lough between the Downhill and Castlerock railway tunnels. The site is defined by dramatic underwater topography featuring large submerged boulders and thick kelp forests. Divers should be prepared for moderate currents, potential surf, and surge, with entanglement hazards present. Due to these conditions, the site is suitable for advanced divers. Shore entry is required to access the reef. Species recorded around this area include harbour seals, grey seals, catsharks, skates, various crab species, and tube worms. Snorkellers may also enjoy the site, which holds a 4/5 rating.

Portnahapple is a sheltered, narrow inlet featuring vertical bedrock walls adorned with dense kelp forests. This site serves as an accessible training location for beginners, offering a typical depth range of 1 to 10 metres with an average of 7 metres. Divers and snorkellers can explore the cove, which includes several small swim-throughs. Access is via the shore, with the main features located approximately 100 metres out. Conditions generally include mild currents and visibility ranging from 5 to 15 metres. Surf may be present at this location. Facilities including parking and toilets are available nearby, and the site is suitable for night diving.