every 5* hotel in doha has pools and clubs.. On friday we spend a day on the beach of the new hotel Grand Hayatt... nice place, was not too much impressed though.. although not too expensive, but it may be because the spa is not open yet..
try Intercontinental hotel.. I think, it is a most convenient beach, with nice spa and bar in the pool.. I also like Ritz, but they still don't have an access to the sea, only pools