Snorkelling in the Manihiki lagoon is excellent without any special effort — wade in from the islet shores and you are immediately among coral, fish, and pearl farm oysters. The visibility regularly exceeds 20 metres and there is no boat traffic to contend with. Black-tip reef sharks patrol the shallower areas; they are generally uninterested in swimmers.
Diving is more rewarding but requires more organisation. The lagoon has good coral structure to depths of 20–30 metres, and the outer reef wall drops off steeply into open ocean — a genuinely dramatic dive for those who can arrange access. There is no commercial dive shop on Manihiki; you need to bring your own equipment or make arrangements well in advance through accommodation contacts in Rarotonga or directly with local families.
The passes through the reef — particularly on the western side — generate current on the tidal shift and concentrate fish. These are the best dive sites, but also the ones that require a local guide who knows the conditions. Do not attempt the outer reef without a guide who knows Manihiki's waters.