If you are searching for something around Tokyo, check this thread :
Beaches accessible from Tokyo [E].
However, you should keep in mind that 1/ July is not necessary the best month for swimming in the sea, as the water can still be cooler than in August, September or even October; 2/ Japan is not a tropical country, and except for some beaches in Okinawa, beaches are usually nice but not amazing.
Japanese Environment Ministry releases on regular basis a list of the "100 best bathing beaches". The ministry added, apart the water quality and landscape features, factors like local environmental education, regional biological diversity and local municipalities' consideration of welfare. The list of those beaches can be found (in Japanese only) on those web sites :
http://www.mori-taki-nagisa.jp/100/beach/index.html
http://www.tabi-link.com/kaisuiyoku.htm