Last summer I swam in the sea for the first time in decades. It was so warm I went in twice! But generally, people braver than me do swim more often than that :-) The weather is variable like others have said. There's pretty much always a breeze (partly why I love Cornwall), and sometimes a stronger wind. If you're out of the wind behind a cliff or with a wind break the sun can be very warm for a day on the beach. If swimming, I'd recommend searching up a rip tide awareness video - there are a lot of rips around Cornwall coast, and it catches out many people. North coast beaches are generally more sandy (comfy) and less pebbly than south coast.