Originally posted by Estyn:

If it were me, I'd take time to gather supplies for another karve, rafts are a deathtrap. And make a bow, and the best arrows you can.

Carefully make your way to your corpse area, and try to lead the serpent away, from as far away as possible, ie, as soon as it's visible. Head for shore, while plinking away with some arrows. If you leave your sail up, your boat will keep moving while you shoot. At least for a time, anyway. If it stalls, grab the tiller and accelerate again.