Buizel, the Sea Weasel Pokemon. Buizel's collar can be used as a flotation device when inflated. Its head stays above water when floating. It spins its two tails like a screw to propel itself through water. The tails also slice clinging seaweed.
Gameplay Tips
Buizel can charge his Neutral Special into four phases.
Buizel's Side Special can be stopped pre-emptively by pressing the attack button after he launched himself. Hitting walls will also make him rebound.
At the end of Buizel's Up Special if you hold the special button then he will start spinning his tail to slow his fall. He automatically goes into this state in the Ex variant.
Trivia
Buizel being able to spin his tail to hover is something that was shown in the anime and was most likely inspired by Tails doing the same.
Buizel was the first of Ash's Pokemon who defeated his evolved form without a rematch.
Ash's Buizel originally belonged to Dawn, but he and Ambipom got traded between the trainers due to the interests of the two Pokémon.
Buizel is part of the Expedition Society in Super Mystery Dungeon where he acts as a mentor to the player and the partner.