Can you do fire arrows?
In my experience shooting a serpent 3-4 times will make it swim away and usually not come back. But doing so in a raft seems high risk.

lizard tail is not very good. Try deer, boar, and if you have it, honey for stamina as a 3 food idea.

Serpent come around in storms and at night or both. Chance is hard to say, you can see them a good way off but even so, its a mix of where it spawns and how close you get to it whether you even know its there or not. Given their tendency to run away, I mostly ignore them, or shoot if I must.