I was on Phuket a fortnight back - Naithon Beach sounds like your ask - check my Phuket page for more info and pix including accommodation and transport. The sections on Kamala and Cape Panwa may be worth checking.
The east coast of Phi Phi has some real nice secluded bays like Relax Beach, Ao Poh, Rantee and Ao Toh Koh. Laen Tong is also great but more high end in accommodation. The nicest beach with easy access to town is Long Beach - can get a bit busy dry season come late November. Got similar info, pix etc on 2 PP pages below which may be of some help to you.