Quick tour of the old city - this was our organized day trip. It takes them so long to get everyone together and on the bus & etc & etc that the guide then zips you around, check, check, check, and you have ten minutes to actually look at what interests you before it's back to the bus. Will return later for a wander around.
For playing water polo. At Cavtat there was one set up in the port, just across the quay from the fishing boat dock. And I saw guys using them, too, just some one-on-one.