Kelp (Ecklonia cava) カジメ
Kelp is a common type of brown algae in colder waters. I think that this is Ecklonia cava which is known as Ecklonia cava in English. There are some subspecies of it. カジメ seems to fit the kelp on this page. Izu 2012
Ecklonia cava at Enoshima. I have snorkeled at this spot many times. It is outside the breakwater where it is hard for shore fisherman to get to. Enoshima 2015
Another picture of the Ecklonia cava. Enoshima 2015
In 2017 Enoshima was hit by waves from a strong typhoon. It did a lot of damage to the parking area and the Ecklonia cava looked like this afterwards. Enoshima 2018