In my neck of the woods, it's Giant Kelp. Unlike the sargasso weeds, it is firmly rooted to the ocean floor, so is a (relatively) shallow-water plant. The root, or "holdfast," is extremely tenacious, as the kelp sees a lot of wave action. It also has floating bladders to hold the long stem upright.

Lengths can get up to 200 feet, but usually about half of that is floating flat on the surface.

It grows fast, sometimes up to 2 feet a day. When I walk on the beaches here, it can sometimes be thick with broken-off fronds. Gets pretty stinky after a while.

These plants are also home to many species specific to them.

The Monterey Bay aquarium has a giant tank with a kelp forest in it.

Now write the damn book already.