Me mate stayed at Copacabana in Jomtien and raves about it.
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but for a real "beach" with warm blue water, go to Koh Larn, if nothing else; a day trip-- very easy to get there by public ferry for 30 baht from Bali Hai pier at the end of Walking Street.
When you go to Koh Larn, best to take sand to the beach, š so to speak--- no real nightlife there. š 555