Fykiaphobia

Fykiaphobia is the fear of and  in open waters. Fykiaphobes can also be afraid of other Aquatic plants (or at least in appearance), such as large arrangements of coral, large beds of algae, and other such slimy growth-like marine life. Fykiaphobes will often refuse to enter open water and dirty ponds.

It is also natural to get unnerved abruptly in a rush while swimming in the beach when your leg touches or get tangled by a seaweed or plastic that can resemble the physical form of the other things in this list.

It is a common fear, especially in children, and is quite high on the list of things that give ocean anxiety. It can give an instant concern when seaweed, kelp or plastic scrapes against your leg or touches you, and you think "What the hell is that!"