I recommend trying the grilled fish
The Sea Green is a small restaurant with good sized balcony just south of the fort ramparts of Galle. Around you, you can see the lighthouse, a small beach, and the rest of the colonial district (declared a UNESCO World Heritage Site). They have an extensive menu, but the service is a bit slow. So there are two options: either order around a half an hour early and go for a walk, or simply order what they have already made, typically fried rice or curries. But you are by the sea, so I recommend trying the grilled fish and grilled shrimp, both of which are very good. You're going to end up eating for less than 5 euros when the same plate might cost 20 in Europe.