The coral off Dadonghai is patchy but real, and visibility runs to ten metres or better outside the summer rainy months. Several PADI-affiliated dive shops on the streets behind the beach run introductory dives, certification courses, and day boats out to Wuzhizhou Island where the diving is significantly better.
A try-dive in the bay runs around 400 yuan; a two-tank day at Wuzhizhou with full gear is more like 800. For snorkelling, the shop boats leave most mornings with masks, fins and a packed lunch. If you have any choice in dates, avoid Chinese national holidays — the diving fleet gets overwhelmed and the visibility crashes.