[1][2] The reserve covers about 15% of Paterson Inlet (Whaka ā Te Wera),[3] a submerged ancient river valley.

[1] The rivers flowing into it drain from pristine undeveloped land with little sediment and nutrient run-off, and warm subtropical and cool Antarctic waters mix in the currents of Stewart Island.

[10] Brachiopod (lamp shells), an ancient filter-feeding shellfish that live on both rock and sediment, thrive in the inlet at depths of less than 20 metres.

[11] Divers also encounter carpet shark, Banded Wrasse, Stewart Island blue cod and lion's mane jellyfish.

Meadows of small red seaweed grow on the sand, helping to stabilise the sediment while providing an important shelter for scallops and surface for spat and larvae.

[4] According to the Department of Conservation, snorkelling is best at a depth of 5 to 12 metres, off the north end of Sydney Cove beach on Ulva Island.