Nearly all the resorts have a restaurant so there's a choice of places to eat and drink on the beaches with several resorts on them. There are also a few dedicated restaurants and bars, mainly on Ao Yai and in the village.
Ao Kwang Peeb and Ao Mook on the other hand are quite isolated with just one resort on each beach. Neither has particularly good food but you're forced to eat there if you don't feel like driving or walking several miles along a dirt track in the dark to find an alternative restaurant in the evenings.
For people on a tight budget, although food and drink prices on Ko Phayam are higher than on the mainland, there are still some simple roadside restaurants where you can get a cheap meal, particularly in the village. Minimarts charge less for beer than bars or resorts and they often have seating and tables outside.
Most resorts have both Thai and Western food on the menu but if you really need a Western food fix then Bluesky and the Buffalo Bay Vacation Club have a lot more choice. (The latter has some excellent Mexican food). Oscar's Bar in the village serves British grub and nearby Multi Kulti restaurant does quite good pizzas. A few bakeries on the island sell cakes and cookies.