Surfing
Hainan has pretty consistent surf most of the year. In the summer, the south winds bring waves to Sanya's south-facing beaches. In the fall through winter, the peak surf season, Hainan's east coast is the place to go. Hainan does not offer world-class waves, but it offers fairly consistent tropical surf, which is almost completely empty.
Water Sports
Yalong Bay, Dadong Bay, and Sanya Bay all have a collection of jet skiing, parasailing, paragliding, snorkeling, scuba diving, glass-bottomed boat, and beach ATV enterprises. Prices aren't generally very negotiable save for scuba and snorkeling, but duration of the ride, however, can. Do not go too cheap on scuba, for obvious safety reasons.