On the eastern seaboard, first beach I would swim at is Sai kaew which is just down the road from me. It is on the Thai Naval Training base and they will charge you 100bht for the privilege, beach pleasant but water clarity not great. My personal choice is Hat Nang Rong at Sattahip Port, again it's on the Navy base there. Drive past Utapao airport on the extension of the 331 and turn left at the checkpoint about 3 or 4km further on. Very nice beach clear water with coral around the island. Busy with Thais at weekends and Bank holidays. 20bht a vehicle and 20bht a falang. In my opinion best beach by some way.