Which Pokémon would be the greatest navigation hazards?

Apart from legendary and mythical creatures, which Pokémon would be the biggest hazards to navigation?

Magikarp is the surprise danger because it can become an enraged Gyarados, while aggressive seabird Pokémon and the enormous but detectable Wailord create different maritime risks.

Magikarp is proposed as a greater danger than its size suggests because a frightened one can become an enraged Gyarados. In the fictional behavior described here, that transformation can threaten every nearby ship. Seabird Pokémon are another candidate because their attack powers compound the aggression of ordinary gulls. Wailord is enormous but comparatively easy to detect with shipboard sensors, and is described as usually passive unless provoked.