What the PP said.
We were concierge this past May on the Wish and didn't get any cabana (we requested both the family beach and Serenity Bay and were shut out) when our booking window opened (we had sent our requests to shoreside at midnight the week prior). We were put on a wait list and about 3 weeks before the cruise we were offered the handicap cabana on the family beach. We ultimately ended up turning it down. Since concierge gets first dibs, I have no idea if a cabana ever became available to non-concierge guests on our cruise.