1 Jamie: You mean we're stuck down here? Did the impact with the bottom of the Loch damage the sub?
2 Adam: Well... No, I don't think so.
2 Jamie: So we can still pump out the ballast tanks?
3 {beat}
4 Jamie: There are ballast tank pumps?
4 Adam: Pumps, yes.

Ballast tanks are used in submarines^[1] to hold either water or air, depending on how buoyant the sub's controller wants the ship to be. To dive, the tanks can be flooded with water, making the sub heavier. And to surface, the tanks can be emptied of water by pumping in compressed air, making the vessel lighter.

[1] Well, most submarines.