Outside the monsoon, the waters around the Perhentians are superb: currents are gentle, and visibility is up to 20m (although sea lice can sometimes be an irritant, inflicting unpleasant but harmless stings). A snorkelling foray around the rocks at the ends of most bays turns up an astonishing array of brightly coloured fish, including black-tip reef sharks, and an occasional turtle. The seas around the islands are part of a national park so the coral is protected, although as elsewhere in the region it suffered bleaching due to high sea temperatures in 2010. It remains to be seen whether that was an isolated incident.
If you just want to explore around the main beaches, then snorkels, masks and fins can be rented from accommodation, dive shops or shacks on the main beaches. Snorkelling trips to undeveloped coves can be arranged at beach stalls (particularly on Long Beach), or most accommodation, for around RM35 per person.
Some very good dive sites lie just a short boat ride offshore, including the Pinnacle (aka Temple of the Sea), T3 and Sugar Wreck (a boat that sank while carrying a cargo of sugar). In addition to fun dives, the islands’ numerous dive shops also offer courses, including Open Water, Advanced Open Water and the introductory Discover Scuba Diving; a handful also offer specialist facilities such as Nitrox. Most places teach PADI courses, although Alu Alu (walualudivers.com) uses SSI certification.