The smooth "finished" surface that your strips get from a thickness planer is irrelevant, because you have to fair the boat anyway, but where a thickness planer does help, is in getting consistent and uniform thickness to feed into your router setup for cutting bead & cove edges. If you have any variability in thickness from the ripping process (ie. the unevenness that results when you pause the feed to shift your hand position) some of the B&C might end up off-center (don't ask me how I know that).

Definitely not worth running out to buy one, if building kayaks is the only use you have for it.