it is best to keep sailing around until you find them. They don't really wander unless chasing the player. They have a chance of spawning under the right conditions as you load new cells, but will essentially just swim in small circles where they spawned unless you notice them and get close enough. The night spawned serpents will also despawn with dawn, so if you dont notice those ones when they do spawn you may never notice them at all. So if farming for them, you have to sail around to get them to spawn to begin with and they will not really go far in search of the player unless they are actively already after the player.