We've hung out on the beach at Kuyaba, when staying on the cliffs. We just bought a couple drinks at the bar and they didn't mind us using their facilities. The security guard even fetched us a couple lounge chairs and towels.
How is the security at Sun Beach? I mean, is it safe to leave a bag on your chair while taking a dip in ocean? I know at some places, you may want to ask the bartender to keep an eye on your belongings when you go off for a swim.