The Castle is one of the three sites featured in the Parks and Wildlife Service brochure for the Governor Island Marine Reserve and is rated by the Marine Life Network guide as one of the best dives in Tasmania. It is formed by two massive granite boulders sitting close together against the reef, with a passage between them that Michael McFadyen describes as more of a tunnel than a cave. The tunnel is about 30 m long at around 25 m depth, and there is an alternative route along the outside of the wall for those who prefer not to go through.

The approach is dramatic. From the reef top the dive drops south over a wall to a large overhang about 30 m long and 1-2 m high, then turns west to the tunnel. Inside, the walls are lined with sea whips, yellow zoanthids and jewel anemones, and the tunnel is known for its crayfish, including some very large animals. Schools of bullseyes and cardinalfish hang in the shade, while banded morwong, long-snouted boarfish and butterfly perch surround the boulders. Large bastard trumpeter, abalone in the corners, small gorgonians and yellow sea spiders complete the list.

Beyond the tunnel is an inverted L-shaped indent filled with large boulders, cracks and further swim-throughs, then a kelp-filled crack leads back up to the reef. In calm seas the dive can finish on Mount Everest, a pinnacle that rises to within about 5 m of the surface and gives a colourful safety stop.

The Castle is a dive most visitors want to repeat, because there is too much to see in one visit. It suits advanced divers comfortable with depth and short overhead sections. The tunnel is wide and has light at both ends, but it should be avoided in surge, and divers should keep their fins off the delicate growth.

Bicheno is on the Tasman Highway (A3), about 2 hr 15 min (180 km) from Hobart and 2 hr (160 km) from Launceston. Boats launch from the small ramp at The Gulch (Waubs Gulch), the narrow channel between Governor Island and the town foreshore off Esplanade; there is trailer parking beside the ramp and fishing co-op. The Governor Island sites are only 2-5 minutes' run out of the Gulch.

Tasmanian boat diving is almost always done as a live-boat drop and pick-up rather than at anchor. The Castle is about 20 m off the eastern side of Governor Island, roughly 3 minutes from the ramp (McFadyen: 41 52 14.1 S, 148 19 02.1 E). Divers drop onto the reef top and are collected after surfacing.

The Governor Island sites face east into the Tasman Sea, so easterly and north-easterly swell over about 1.5 m, or a fresh easterly/north-easterly sea breeze, makes them rough and surgy and can close the Gulch entrance. Westerly and north-westerly winds blow offshore and give the flattest seas; southerly swell wraps in and stirs the bottom, and a big southerly can drop visibility under 2 m for days. Winter (June-September) usually brings the clearest water (20-30 m) and most settled westerly weather; summer afternoons are often spoiled by the NE sea breeze, so dive mornings.

Avoid the tunnel in any real easterly swell, as surge runs through it.

Descend to the reef top, head south over the wall and follow the 30 m-long overhang, then turn west to the tunnel entrance at about 25 m. Swim the tunnel (or the outside wall route), explore the inverted-L boulder field beyond, then follow a kelp-filled crack east and circle the reef clockwise to Mount Everest, finishing at 5 m. McFadyen limited deep time to about 25 minutes of a 40-minute dive.

Tunnel is an open swim-through with light at both ends; no line needed, but a torch helps.

No confirmed full-time commercial dive operator in Bicheno as of 2026: the long-running Bicheno Dive Centre was reported closed (Michael McFadyen) although some tourism pages still list it - verify before travelling. Sites are dived by club boats (Tas Uni Dive Club, Tasmanian Scuba Diving Club, Leven Scuba Club), the annual Combined Clubs Weekend each June, and occasional trips from Launceston (Tasmanian Divers) and Hobart (Go Dive Tasmania) shops.