How do I politely deal with the constant beach hawkers on the south coast

Friendly directness works best. A smile, a small head wave, and a clear "no thank you" or simply "epa, sthuthi" (no, thanks in Sinhala) ends 90 percent of approaches. Avoid pretending you do not hear, that often makes hawkers follow you. Reasonable yes-to value: fresh young coconut (200 to 300 LKR is fair), grilled fresh fish or seafood from beachside stands at lunchtime, and ad-hoc surf lessons from someone the local guesthouse vouches for. Skip: "massage" offers on the beach itself (often unqualified, sometimes uncomfortable), "I will show you a special turtle place" (commission), "free spice garden tour", and offers from people who follow you. Generally, the hawkers in front of established beach restaurants are repeat business and behave better than the roaming ones.