You begin the game as the ship's captain, determined to rescue the other people, presumably because "lost all crew and passengers in an alternate dimension" doesn't look real good on a resume, and you're going to need to get your hands dirty to do it.
Use [WASD] or the [arrow] keys to move, jump, and swim, while tapping [spacebar] attacks, though the controls are completely customiseable from the pause menu. Each area is filled with coins and more to collect and baddies to wallop, and though some itty-bitty enemies can be taken down with a single whack, others will need more to bring them down. Each customer you rescue is then unlocked to play as, and since they all have their own different abilities, replaying levels as someone else can help you unlock new areas or reach something you previously couldn't!