Mariposa Reef is a shallow reef north of Four Mile Creek that the Marine Life Network guide rates 5 out of 10 as a fairly easy shore dive for people holidaying on the coast. It involves a 100 m beach walk and a snorkel out to a small island, with depths of 3-9 m.

The guide lists crayfish, abalone, Australian salmon, reef fish and kelp beds. It is popular with fishermen, and poor visibility is common, so it is best on calm days and on the northern side of the island.

It suits holidaying divers and snorkellers staying at Scamander or Four Mile Creek.

The north-east coast from St Helens to the Bay of Fires is built of the same orange-lichen granite as Freycinet, and under water it forms rounded boulders, cracks and swim-throughs. The regional guide notes damage from the long-spined black sea urchin (Centrostephanus rodgersii) on several sites; this urchin has spread south with the warming East Australian Current and grazes kelp and weed down to bare 'barrens', so expect patches of bare rock among the weed.

The north-east sees far fewer divers than Bicheno or the Tasman Peninsula. There is no regular charter boat, so most diving is done by Launceston club members and visitors towing their own boats, and conditions are checked on the morning rather than booked in advance. The nearest hyperbaric chamber is the Royal Hobart Hospital, more than three hours' drive away; Launceston General Hospital has no chamber, so evacuation planning and oxygen on board matter here.

Four Mile Creek is on the Tasman Highway between Scamander and St Marys turn-off, about 25 minutes south of St Helens. Park at the beach access north of Four Mile Creek, walk about 100 m along the beach and snorkel out to the small island and reef. Allow about 2 hours' drive from Launceston via Scottsdale or the Tasman Highway through St Marys.

Park close to the water where possible and check the exit before entering; the granite is slippery when wet and swell surges can be deceptive.

The St Helens and Bay of Fires coast faces east and north-east into the Tasman Sea, so easterly and north-easterly swell over about 1.5 m, and the common summer NE sea breeze, make it rough; the Georges Bay bar breaks in easterly swell. Westerly and south-westerly winds blow offshore and give the flattest seas and clearest water. Southerly swell wraps around St Helens Point.

Visibility falls after rain when Georges Bay drains tannin-stained water on the ebb. Poor visibility is common; the regional guide recommends calm days and the northern side of the island.

Snorkel out to the island, descend on the northern side at 4-6 m and explore the kelp beds and reef out to 9 m before returning to the beach. Turn at a third of gas to allow for the swim back and a possibly harder exit, finish with a 3-minute safety stop at about 5 m if the depth allows, and exit where you entered unless an alternative exit has been checked beforehand.

East Coast Scuba Centre (124 Binalong Bay Road, St Helens) is listed by Wikivoyage as offering dive and snorkel equipment hire - current trading status and whether it runs boat dives are unconfirmed. Otherwise sites are dived from private boats and by Launceston clubs and Tasmanian Divers (Launceston) on occasional trips.