Buy clothing vacuum packs, and a small vacuum cleaner for use on the boat,
and to suck out the air from the packs. You will gain about 300% space
compared to loose clothing.

Even plain old ziploc bags are great. Roll or otherwise fold the clothes,
put them in the zipper bags and squash 'em a bit to get the air out. Saves
you space and keeps them fresh and dry. I keep a ready supply of several
sizes. The small sandwich ones are great in a pinch for all sorts of tiny
bits (cash, wallet, phone, etc). The gallon XL, XXL and XXXL sizes have
come in handy on many occasions and are usually cheap to buy in bulk at
Costco-type stores.