Grindal Island is an 81-hectare, 26 m-high island in Lincoln National Park, among the scatter of islands in Thorny Passage south of Port Lincoln. The Marine Life Society describes a jetty on the sheltered west side and deeper reefs on the south side, reaching about 20 m, with large boulders, bommies and swim-throughs on a rocky bottom covered in seagrass and kelp. The island is best known to visitors as an alternative venue for Port Lincoln's sea lion swims: Calypso Star Charters takes guests to Hopkins, Grindal or Blyth Island depending on which offers shelter on the day.

Australian sea lions haul out on the island and enter the water to investigate swimmers, behaving much as they do at Hopkins. For scuba divers, the south-side reefs offer a varied temperate dive. Schools of sweep and silver drummer move over the boulders, large western blue groper cruise the reef, and the bommies and swim-throughs are coated in sponges and algae.

The Society warns that visibility is sometimes poor, as tidal streams through Thorny Passage can carry sediment. Grindal suits snorkellers on tour and Open Water divers with a boat. It combines well with nearby Memory Cove, Taylors Island or Lewis Island for a day of island diving in Lincoln National Park.

The sheltered jetty side is handy for a surface interval, and the island's position means it often offers a lee when south-westerly swell is running through the outer passages. As with all sea lion encounters, keep movements slow, let the animals initiate contact and give any large bull a wide berth. Tidal streams can run strongly around the island's points, so plan deeper dives for slack water and carry an SMB.

The combination of an easy sea lion swim and a proper boulder dive makes Grindal a good all-round island for mixed groups of snorkellers and divers.

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/Kirton Point ramps, running south through Boston Bay and Thorny Passage (about 45-60 minutes). Calypso Star's sea lion tours use Grindal when Hopkins Island is exposed.

Private boats should anchor off the sheltered west side.

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 notes visibility at Grindal is sometimes poor.

Snorkel with sea lions in the sheltered water; divers head to the south-side reefs, weaving through boulders, bommies and swim-throughs at 12-20 m before returning to the shallows.

Calypso Star Charters sea lion tours (as an alternative to Hopkins/Blyth); Port Lincoln dive shops and private boats for scuba on the south-side reefs.