Make sure zippers are either waterproof or covered by a flap of fabric. A high neck will keep you warm, and ample pockets give you places to stow a flashlight, knife or snack.
Instead of cotton, opt for wool or synthetics like fleece or polyester that will continue to insulate even when wet. A light jacket that can be worn alone or under your waterproof shell is a particularly useful item.
Protect legs from rain and spray and your rear from wet cockpit benches with waterproof trousers. They come in waist-high versions, as bibs with adjustable shoulder straps, or as salopettes that act as pants and a vest all in one.
Keep your feet dry in sea boots with grippy, nonmarking soles. Make sure to get a pair big enough to accommodate wool or synthetic socks.