The Evil Ocean was an ocean monster that appeared in the form of a huge wave. The only notable feature of the Evil Ocean is its evil-looking face; otherwise, it looks just like a normal ocean wave. In a flashback in the episode "His Hero," the Evil Ocean was destroyed by Billy.
Its face is similar to the face of the Fear Feaster. The Evil Ocean appears to be a tidal wave, since even Billy was shorter than it. It has no arms or legs.