I GOOGLED: various combinations of the terms "lost at sea" "salt water/sea water," "boil/blister/burn," "skin damage," and "shipwreck"
I LEARNED: Most of what I found were recommending salt water for first aid or beauty reasons, or were about chemistry/physics. :-/ A couple sites pointed out that the salt water will dry out skin, which might lead to redness, chapping and cracking, but didn't offer a lot of details. I seem to remember seeing a documentary once about WW2 sailors lost at sea in the Pacific for days, and one of the survivors talked about how horribly raw his skin was, but I don't know if that was just from the sea water or a combination of salt and sunburn.
I'm just trying to figure out what their condition would be when help finally did arrive--I know they're going to be pretty exhausted and probably hypothermic, but just how bad will their skin be, especially hands and feet? What could they be doing to protect themselves while they're waiting for rescue?