Since you're playing shag type beach music I would say to stick to roots/dancehall reggae. Bob Marley of course, I recommend the "Exodus" and "Kaya" albums. "Legend" has most of the Marley hits everyone knows. Check out Steel Pulse, Jimmy Cliff, Bunny Wailer, Peter Tosh, Shabba Ranks, Luciano.
If you want to throw something more modern/upbeat in look into Damian or Ziggy Marley, Sean Paul, Sizzla, Elephant Man, Beenie Man, even Matisyahu.
I say buy some albums or download some songs to see what you like. I have tons of reggae and would be happy to send you some samples if you want to try before you buy )