The Underwater Scooter is an item that will allow you to swim freely around the ocean. The Underwater Scooter has a limited amount of power to be used in it; ~16 seconds. To use the Underwater Scooter, assign it to one of your hotkeys, press the corresponding key, then click and hold the left mouse button or space key to accelerate. Not having the scooter active (or otherwise draining the battery) will allow the scooter to charge itself. It's recommended to have a backup scooter when mining for when the first one runs out of power.
The Underwater Scooter can be very useful when mining in deep sea areas, since you are able to propell yourself back up the steep cliff that you fall down from to get to the various rare ore deposits. It can be quite difficult to make it back to base up without the Underwater Scooter, since the player is only allotted so many jumps before they must rest to replenish their meter.