Hi! I have been to 2 beaches near Tokyo, Enoshima beach and Chigasaki beach. I think that there is more to do around the Enoshima area (close to Kamakura and Enoshima island has attractions too), but the beach itself is better in Chigasaki. I found the water to be clear and clean there, the beach is clean too (not so much seaweed) and when I was there the waves were also not that high meaning it was very comfortable to float around on my inflatable tube :)
Have a good summer!