I think almost no one from Europe expect to swim in Japan during his or her short holiday in Japan but there are plenty of nice beaches throughout Japan. Okinawa prefecture has the best beaches in Japan thanks to its climate, sub-tropical, but many local Japanese enjoying the beaches near their home towns.
Except in Okinama, the beach season starts in June or July and ends in late August, as same as the beaches in Europe but the difference is that there is no one staying near the beach a week in Japan. Interest thing is that there is not so many hotels near the beaches except in Okinawa. People usually come to the beach for a day.
By the way, there is no nude beach in Japan and it is illegal to be naked at public places. Therefore if you do not have a swimming wear with you, do not try to swim at the beach.