Ko Yao Yai's only dive school is an on-the-ball operator covering a range of PADI courses, two-dive day trips locally and to Ko Phi-Phi, and liveaboards to Hin Daeng, Hin Muang and the Similans (22,900B), plus snorkelling excursions to Ao Phang-Nga, Krabi and Ko Phi-Phi. If you're staying on Ko Yao Noi, they'll help with transfers.