Odaiba is the only beach within Tokyo city limits. It's an artificial beach that's about 800 meters long. The beach is beside Odaiba Seaside Park. Odaiba is in a relatively central location in Tokyo. This beach gets very crowded at the summer peak. You can swim, play beach volleyball or just get some sun.
Chiba prefecture is about 30 minutes to an hour from central Tokyo. Chiba has dozens of excellent beaches. It's a popular place to surf (the waves get big here).
Kamakura is 30 minutes from Tokyo and has several beaches that are popular with young people and surfers.
For more pristine beaches check out the many beaches in nearby Izu (about 2 hours by train or car).