Probably everything was addressed already but here my short list of suggestions:

1. Don't go into deep ocean with raft.
2. If you really have to, go there during the day and clear weather (sea serpent only appears at night and/or during the rain)
3. Remove the sails when wind is "grey" (blowing towards you), that way you go faster... well, little bit...
4. Try to do short crosses between land masses and sail along the shores.
5. Do have stamina food ready in case you feel like raft will go down. If you're close to the shore, you should be able to swim to land... and pray it's not a swamp or plains :D