Temperature on the Sea Island trip is something I am giving a lot of thought too. Average temperatures for March in the South Carolina/Georgia area are 40 degrees at night and 60 during the day. That's the average, it can certainly get much colder or warmer than that on any given day/night. I expect I will be wearing a combination of the dry suit and thermals for much of the first week or two of the trip.
Driving home through the neighborhood this afternoon, with SPARTINA in tow, I saw a friend out walking, training for the Appalachian Trail. He did the 2,200 mile trail a few years ago and is getting ready to do it again. He is very technical in his approach to hiking and we have often talked about dealing with the elements in respect to both hiking and small boat sailing. I pulled over this afternoon to ask him about how he dealt with the cold and the wet while hiking the trail (he started his first trail in two feet of snow). His reply was simple: Don't get cold, don't get wet. Use the right gear, use your experience and good judgement. Chance of rain? Be ready for it, don't wait until it rains to put waterproof gear on. Chance of cold, be layered up. It is a lot easier to be a little warm than too cold. And make sure you have good warm clothes for sleeping.
That all makes sense to me.