Yeah, staying away from swamps at night is a good idea.

The ocean is actually better, since like Fzanco said the serpents can be outrun if you do encounter one.
You may not even encounter one; I don't know what the spawn chance is, but it definitely isn't 100%.

Plains are nasty though, for sure. In fact, I thought (before opening this) that the thread title was referring to the mosquitoes. :D

PS. What I sometimes do is just float in one spot through a night or a fog, if I think it's too risky to keep sailing. Have yet to get attacked while still.