Wanning's east-facing coast catches more rainfall than the sheltered south coast around Sanya, so the wet season matters more here. October through April is the dry period and the most reliable for surfing and outdoor activity; May through September brings heavier rain and intermittent typhoon risk, though the surf is often bigger during tropical swells.
The Xinglong area is sheltered by the mountains and is usable year-round, making it a reasonable wet-weather alternative when the coast is unpleasant. The China Surf Open, when held at Riyue Bay, draws a crowd and changes the beach atmosphere — worth timing around if that's not what you're after.