Both Kugenuma Kaigan and Shonan Kaigan are very well-known as surfing beach. You can see people surfing all the time but winter; since spring to autumn. More photos.
I went Kugenuma Kaigan for the first time with my colleagues. It took about one hour by train from Yokohama, and it takes 25 minutes walk from 鵠沼海岸駅(Kugenuma Kaigan Station).
The day was not enough to go into the water; I was wearing swimming suits though. However, I enjoyed its summer and beach air a lot. Amazingly, there were so many crams in the sand, of course alive ones, and we even could see fish swimming at the shallows. Numerous seagulls were hunting those fish. That was pretty much summer beach. Here is interesting movie I took. A cram is escaping into the sand!
On August, there will be people at Kugenuma Kaigan and Shonan Kaigan. I know Shonan Kaigan is so famous and be crowded a lot, just like as no place to even sit down! However, Kugenuma has much room to relax and time your own beach time. You can check what Kugenuma Kaigan is like with live camera.