A seaweed monster appears in "No Sharking Zone", an episode of The New Scooby and Scrappy-Doo Show. It was the disguise of Cowabunga Carlyle.
The monster resembles a large humanoid mass of dark green seaweed, with red eyes.
Cowabunga Carlyle used the disguise to scare away surfers while he searched for the lost treasure of Scareruba. His other disguise was a submarine made to look like a great white shark.