There are two popular swim spots in book town Hay on Wye, Glasbury Bridge and the Warren. There's nothing romantic about the bridge itself, but it's right next to town, there's easy access from a big pebble beach where children can paddle in the shallows and there are endless stones for skimming.
A popular putting-in spot for kayakers so you may have safety in company. A good long inland beach with a pleasant current and not too deep.
Fun to swim against it or with it and although it is near to town and with many sunny acres adjoining, the unfarmed forgotten-about style gives it a secret feeling.