How do I stop myself from being slowed down in the Sea?

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So apparently being underwater makes it hard to move, and it is slowing me down. I know this because I get a message at the end of every fight telling me so. How do I stop from being slowed down? What effects does being slowed down have on me?
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alec benjamin, alessia cara - let me down slowly (slowed down)
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Kei Scampa, SHVETS production, SHVETS production, Yan Krukov