Many salts, including soduim chloride, are hydroscopic. That means that they will capture moisture out of the air as long as the relative humidity is above some minimum value (for that particular salt) and remain damp, instead of drying out completely. A semipermeable membrane (like skin) will develop a fluid pressure in the direction of diluting a higher salt content (osmosis). So if the salt concentration on the outside your skin in higher than the concentration on the inside, some water will be pumped from inside to outside (in the direction that it is always moving unless you are submerged in fresh water, in which case, your skin pumps water into your blood stream). And this water will also add to your skin moisture, until it evaporates.

By the way, swimming (vigorous activity under water for extended periods) in any water, fresh or salt, "dries" your skin by emulsifying the surface oil and floating it away. This is the same way that excessive scrubbing "dries" your skin. Normal skin has two components that lubricate and soften it and affect its permeability in two very different ways: water content and oil content. Most people use the term "moisture" (and the result of moisturizers) to refer to any combination of these two very different skin fluids.

Too much salt will indeed deydrate you, but if you ever go and see a doctor about being dehydrated, the first thing they tell you to do is drink water with a little salt mixed in. When someone is severely dehydrated, they'll inject saline solution (basically salt water) into your blood stream.

In the past two days I've been drinking 2 or 3 glasses of water with a little salt sprinkled in, per day. I've found that it has helped my dry skin, and even my lips are less dry. Certainly, they're not cracking up the way they were 3 days ago.