Generally in May the beaches are still in use.The only time the west facing beaches are not usable is if there is a major storm coming in.Several beaches have notorious 'rip tides' (Karon and Surin) so pay attention to the beach warning flags.
For value for money and good night life Patong Beach will probable be the best for you.
Diving and snorkelling are best on Phi Phi at this time but the day trip dives from Phuket are still good.
