Salt water is well known as an antiseptic. Soap is well known as a pollutant. Therefore save your skin and save the oceans by heating a saucpan full of seawater to a comfortable temperature and shave using only that to moisten your babysbumsmooth cheeks.
After shaving, take a couple of dabs of olive oil from the galley (you do have olive oil in the galley don't you? [see cooking thread on this forum for further details]) and use this to remoisturise your undoubtedly healthily-glowing complexion.
Alternatively, you can, of course, grow a beard, but beards on young mean can look very pretentious. Really, beards should be reserved for sexogenarians and those above them.
Never tried it for shaving but the soft soap that goes into pump type soap dispensers lathers pretty well in salt water. Kinda slippery stuff though when it hits the salt too, so I think it might be better than the real stuff in fresh it being so nicely lubricating like.