Lewis Island is one of the Thorny Passage islands south of Port Lincoln, and the Marine Life Society describes it as principally a seal colony dive in about 10 m, with nice bommies off the south-west tip and very colourful marine life. It adds that visibility is moderately poor and currents very strong in the deeper locations. Seals from the colony – Australian sea lions and fur seals occur in this area – often join divers in the shallows, swooping and blowing bubbles.

The bommies off the south-west tip are covered in sponges, ascidians and algae, with sweep, old wives, magpie perch, blue devils and western blue groper among them. Currents through Thorny Passage are the main complication: the deeper areas should be dived at slack water, with a tender following divers' SMBs. Visibility tends to be lower here than at the outer islands.

Lewis Island suits experienced divers with a boat who want a seal encounter without travelling to the Neptunes or paying for a tour, and it combines with Taylors Island and Memory Cove for a Thorny Passage day. Seal encounters are most rewarding when divers settle quietly on the bottom in the shallows rather than chasing the animals, and summer and autumn, when young seals are active, are usually the liveliest times. Because visibility can be limited, stay close to your buddy and the reef, and use a surface marker so the boat can track you in the current.

As with all seal colonies in South Australia, keep dives compact and be alert for white sharks.

Port Lincoln is about 650 km (7 hours) by road from Adelaide via Port Augusta and the Lincoln Highway, or a 45-50 minute flight (Rex/QantasLink). Boat access from Lincoln Cove Marina or the Tulka ramp south through Thorny Passage (about 45-60 minutes). Anchor in the lee of the island near the seal colony or off the south-west tip for the bommies.

Boston Bay and the waters around Port Lincoln are sheltered from Southern Ocean swell by Boston Island and the Lincoln National Park coast, so the town jetties can be dived in most weather; strong south-easterly and easterly winds are what chop them up. The islands of Thorny Passage and the lower Eyre coast are progressively more exposed to south-westerly swell and strong tidal streams, and need swell under about 1.5-2 m and light winds. Winter (May-August) brings frequent cold fronts with big south-westerly swell but also calm, clear spells; summer has more reliable weather but regular afternoon south-easterly sea breezes.

Water is cooler than the gulfs, around 13-15°C in winter and 17-20°C in summer. The Marine Life Society warns of moderately poor visibility and very strong currents in deeper locations here.

Dive the shallow water near the seal colony and let seals approach, or drop onto the bommies off the south-west tip at slack water and explore at 8-12 m.

No regular operator; private boats and occasional club trips.