I've always preferred natural fabrics so most of my clothing is cotton or wool. Long underwear and socks as well as other technical clothing is generally synthetic. Day-to-day living on the dock is pretty much the same as ashore.
Dinghy runs usually mean some spray so I am more careful about fabric choices in the dinghy. Still, I haven't had any issues with cotton t-shirts -- if it's going to be that wet I wear my foul weather jacket at least anyway. I don't like having a wet butt regardless of the material type so I slow down rather than get wet.
I wouldn't buy all new clothes (unless your looking for an excuse) until you get some experience living aboard and determine what works for you.
I think Dave's right, especially that day-to-day living at the dock is a lot like on land, and I should have been more specific about cotton. 2 places I *won't* wear cotton - (1) sticky-hot places like hiking in the Virgin Islands (it holds sweat and B.O. and just stays moist all day) and (2) cold rain or spray. OTOH, Hanging out in the Bahamas, in winter, temps about 70 - cotton T-shirts are no problem.
We often wear loose lightweight long cotton pants - surprisingly, they can be cooler than shorts in some conditions. If "wet dinghy butt" is going to be an issue, we either ride standing up or if its too rough for that, we go ashore in full foulies - which earns us some funny looks in the grocery store if its a sunny day.
BTW, Serendip, welcome to SN, our hailing port is Northport, MI and we still miss the Sweetwater Sea.