Bamelin

Diamond: It has a flotation sac that is like an inflatable collar. It floats on water with its head out.

Pearl +2: It swims by rotating its two tails like a screw. When it dives, its flotation sac collapses.

Platinum +2: It spins its two tails like a screw to propel itself through water. The tails also slice clinging seaweed.

HeartGold / SoulSilver: It inflates its flotation sac, keeping its face above water in order to watch for prey movement.

Black 2 +3: It inflates the flotation sac around its neck and pokes its head out of the water to see what is going on.

Legends: Arceus: It moves freely in the water by spinning its forked tail for propulsion. The resemblance to the screw of a steamboat is coincidental.