From the Super Mario Wiki
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The Steel Diver is a long-range projectile weapon/item introduced in Super Smash Bros. for Nintendo 3DS / Wii U. This weapon is based off the appearance of the Blue Shark submarine from Nintendo's Steel Diver game for the Nintendo 3DS. It launches hard-to-see torpedoes, that start out slowly before they speed up and explode upon contact with an opponent, an item or if they reach their maximum distance.