The islands off Cape Le Grand National Park, including Thomas Island, form part of the Recherche Archipelago's inner group east of Esperance.

The islands are granite domes rising from sand and reef, with the typical Recherche habitat: boulders, gutters and overhangs covered in kelp, sponges, gorgonians and encrusting corals, and fish such as blue groper, queen snapper, harlequin fish, boarfish, sweep and bullseyes. Several islands in the archipelago host Australian sea lion and New Zealand fur seal colonies, and the Sanko Harvest oil spill of 1991 affected seal colonies in this area.

Diving here is by private boat or by arrangement with the Esperance dive shop, and the long run and exposure mean trips are weather-dependent. For better-documented Recherche diving, see Remark and Frederick Islands, Long Island, Woody Island and the Sanko Harvest.

Esperance is about 720 km (8 hours) south-east of Perth via the Albany or Great Eastern and Coolgardie-Esperance Highways, with daily flights from Perth. Dive boats and private boats launch from the Bandy Creek Boat Harbour (5 km east of town, with ramps, parking and toilets) or operate from the town waterfront on The Esplanade. Air, nitrox and hire gear are available from Esperance Diving & Fishing on The Esplanade.

Thomas Island and nearby islands lie off Cape Le Grand, roughly 40-60 minutes by boat from Bandy Creek. Trips are by arrangement with Esperance Diving & Fishing or by private boat; Woody Island Eco Tours' archipelago cruises pass some of these islands for wildlife viewing.

The Recherche Archipelago lies open to the Southern Ocean, and the islands shelter each other only partly. Boat diving needs swell under about 2 m and winds under 15-20 knots; the prevailing south-westerly swell and winter cold fronts cancel many trips from May to September. Summer brings reliable morning calms but strong easterly sea breezes most afternoons.

The islands off Cape Le Grand are more exposed than Woody Island and need swell under about 1.5-2 m for a safe anchorage in the lee.

Anchor in the lee of the chosen island, descend to 12-18 m and follow the granite boulders and gutters, returning shallow; spend the end of the dive near any seal haul-out.

Esperance Diving & Fishing (72 The Esplanade, Esperance; operating since 1983) – boat dives in the Recherche Archipelago on demand/weather permitting, minimum numbers apply for long trips – by arrangement.