Japan has around 1 million surfers (although many are not active at any given time). There are hundreds of beach and reef breaks all over Japan. Generally the east coast of Honshu (the main island of Japan) is the best place to surf. There are good spots in Sendai, Fukushima and Ibaraki. The many fine beaches of Chiba, Kanagawa and Izu are within easy reach of Tokyo.
Japan has many beach breaks. Swells are normally small (nothing overhead most of the time). Annually, about 30 typhoons in the pacific pick things up. Each year 7 or 8 typhoons hit Japan directly — making the waves as big as you want.
Japan is surfable all year with a full wetsuit (with the possible exception of Hokkaido). There are plenty of fair weather surfers (crowds die down in the winter). The one exception is the ever popular Shonan. Here's a crowded tiny wave in the middle of winter.
In the summer most beaches are closed to surfers from about 8 AM to 4 PM. During this time the beaches are invaded with vacationers. Some beaches are popular with families while other spots attract a younger set.
There are many female bodyboarders who appear in the summer months. Few men bodyboard in Japan. Some Japanese women also surf (some even make it out in the harsh winter months).