Great British beach activities: 10 best sea swims

For lagoon bathing

The Blue Lagoon at Abereiddy, Pembrokeshire

This Pembrokeshire beach, with its 470-million-year-old rock forms, has pebbles and sand made from pounded grey slate. The slate gives a  deep-blue colour to the water in the lagoon. It's largely cut off from the ebb and flow of the sea, but be careful - there are no lifeguards, and its west-facing direction means that a large swell will create turbulence. If you're a confident sea swimmer, this is for you (www.goodbeachguide.co.uk).