I killed 3 of them in one day (it was the night, to be honest). First, i sailed in rain (it doubles your chances) with only half of speed, within the shore sight, but in the ocean. When serpent appears, it likely follow you for half of minute (and you cant' see it because of rain, waves and distance) and then leave. When you find it, you need to attack it or allow it to bite your boat. And then rush to shore and kill it with bow.