If your dino gets stunlocked by these, then get on a flying Mount, whistle for the stunlocked dino to follow, move close to the render distance but so that you are still able see it. It might be the dino has of a certain size, or atleast bigger than the cnidras but the cnidras will unrender, and your dino will be free to swim away ! :)

More Cnidaria Encountering Tips