Japan is an island country with a long coastline. Naturally, it boasts many picturesque beaches. The warm season arrives first to Okinawa in the south. The water becomes good for swimming in early April or even late March. Yonaha Maehama on Miyako island is considered Japan's top beach in the south. Also, it's worth a visit to Nishihama on Hateruma Island or Kondoi on Taketomi Island. There are a few good spots near Tokyo, in particular, Odaiba or Izu peninsula beaches.
The official start of swimming season in Japan happens only in mid-July. During the Umi Biraki ceremony the sea is blessed and becomes available for swimming. However, you don't have to wait that long. At many beaches you can swim before Umi Biraki. At this time you'll certainly see fewer people on the beach.