It was a kind of custom. I was enjoying marine sports like water-skiing or wet bike during a summer and, according to a definition of my friends, the season must be from the first of May to October. Indeed, it is six months long summer. Exactly saying, we preferred a lake close to Mt.Fuji rather than sea because sometimes it shows us a completely flat face like a mirror. One of drawbacks to go there was it was too cold in May. The water was just made from snow.