Snorkelling is of course a draw on Kapas; most places to stay can rent out gear, or arrange a boat trip out to a choice site. Visibility is best between May and August, but jellyfish can be a nuisance in June and July. Some of the most popular snorkelling spots are around rocky Pulau Gemia. If you’re just renting equipment then try the rocks at the edges of the beaches beyond Qimi Chalet and the campsite.
Diving isn’t generally considered to be as good as on the Perhentians or Redang, but there are opportunities, particularly on the eastern side of the island. Popular sites include Berakit Reef, Octopus Reef and Coral Garden. Blacktip reef sharks are sometimes seen, and you can find turtles at Coral Garden. There’s only one dive shop, Aqua-Sport (wdivekapas.com), which offers PADI courses as well as regular dives.