MARILL's oil-filled tail acts much like a life preserver. If you see just its tail bobbing on the water's surface, it's a sure indication that this POKéMON is diving beneath the water to feed on aquatic plants.
Sapphire:
When fishing for food at the edge of a fast-running stream, MARILL wraps its tail around the trunk of a tree. This POKéMON's tail is flexible and configured to stretch.
Fire Red:
The end of its tail serves as a buoy that keeps it from drowning, even in a vicious current.
Leaf Green:
The tip of its tail, which contains oil that is lighter than water, lets it swim without drowning.