This assumes you've got all the regular cold-weather boating stuff and are bringing lotion, good libations, gas log fire, decent camp amenities, etc.
I second the thermos. Fill with hot tea in the am and it'll be gone by the time you get to camp whether anyone swims or not. I like a thermos of red Roobios tea dosed with honey and half and half so it's got some calories and fat to help keep the chill off.
Put one or two on each boat or, better yet, make them required equipment for everyone on the trip.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse