Salt
Cotton does feel nicer. We use lightweight fleece blankets and bring a topsheet from home on the boat. Folded in half lengthwise, the sheet acts like a sleeping bag and serves as both top and bottom sheet in a standard bunk. After a while in salt air however, just about any fabirc will tend to absorb more and more moisture, and not dry out as well as you might like because of the salt adding to its absorbtion qualities and slowing down its drying. Hydroscopic is the term that comes to mind. Salt will get into nylon too, and make it feel clammy as well. Since the salt affects both cotton and nylon, we opt for feeling a bit more comforable with cotton for at least a little while before the clammy cling sets in. Having separate pieces and less material than an entire sleeping bag also makes it easier to rinse out the salt and dry the bedclothes when the time comes to do that. One of the advantages of sailing in Wisconsin is not having so much salt around.