Originally posted by blankitosonic:

hmm right now , only when its a night spawned serpent and a new day its starting , before it was when hp gets low but was changed not so much ago , you now have to keep the agro on the serpent

But the serpent definitely fled on low health while it was on the sea. But when I went after it and aggroed again it didnt flee at all. I think i killed it shortly before the new day so i didn't get to check that part.
But an even earlier serpent that I fought before getting the harpoon also followed me near the shore where it started fighting greydwarves while compeletely refusing to flee.
So in other words, serpents always seem to follow you to to the shore so you only need to harpoon them once you get there.