a Tokyo beach?

Konnichiwa Mina-san!
Can you suggest me a nice beach/place where I can swim in the sea anywhere near Tokyo? Or the best - whatever - swimming pool? Or an onsen with a bit COLDER water for a hottest day (do people stay in HOT onsens in summer?) I was told Enoshima beaches are OK. What do you think about it? I just need a cool clean water...
Thanks for your help!

Depends how near from Tokyo it is. Enoshima is about 1,5 h away by train and is extremely touristy, not so nice and clean.

They are lots of beaches (with cooler water !) off the coast of Chiba, on the Pacific Ocean. It will take you more than 2h from Tokyo to reach them however. Don't expect palm trees. Big waves for surfing, but not landscape.

Thank you!
I am planning to go to Chiba, anyway... but anything closer to Minato-ku (South-East Tokyo) where I will stay?