I feel like an idiot for even having to ask, but after dying three times to this and having to restart all the way back from the tutorial save point I'm not going to trial-and-error my way through that again.

So, uhm... when swimming, you press the sprint (shift) button to swim, and the aim (right mouse) to go underwater, but, uhm... how do you swim back up to the surface? Because jump\up (space) isn't working, and neither is simply pointing up as I swim.