These are generally called kelps. This image shows submerged brown algae. During storms, kelps wash up on beaches. In living condition, kelps attach to the bottom of the ocean by a portion called the holdfast. The image shows a holdfast.
Kelps can get very large.
Some are over 300 feet long. When they wash inshore after a storm, their size is still impressive. Kelps are harvested as cattle food or to extract minerals and the components of their cell walls, a gummy material called algin. Algin is used commercially as a stabilizer in food products or shaving cream or cold cream. Red algae are less common along the coast of California but they are present.