Not exactly a few, is the marine equivalent to leaves falling from the trees at fall. There are mats lying in the beaches each september. Some of them are very thick.

On the other hand, can't help to mention that Neptune balls is a bad choice for something that is not normally spherical and is made by Posidonia, a creature named in honor of Poseidon. (Ironically there is also a Neptunia plant, and their flowers are shaped as balls).