Koh Jum/railay/koh Lipe Etc (ko Pu)

sorry if ive already asked these questions but im still undeceided with only a feew days beforei travel.

either railay,koh jum or somewhere else.ko lipe possibly although ive heard that gets crowded,i dont mind crowds just so long as the beach isnt crowded and i have my own space.

what i want is nice sandy beach with good clean deep water ideal for swimming,quiet as possible away from crowds etc.

koh jum sounds nice and chilled but beaches sound a bit crap ,very rocky etc.

anyone got any reccommendations,its only for 3 nights over the christmas peroid

any help appreciated