My luck finding serpents seems to have run its course. They are no longer at any of the familiar haunts I have marked on my map. In the past 10+ days I have come across 2, and 1 of them got away, after trying to drag it to shore and getting mobbed by fulings, leeches and draugr's. They actually sank my vessel, which has only happened once before. My new strategy is if I run into them, I run into them. I always sail with the harpoon in the action bar and plenty of frost arrows in my quiver.