You could drag the serpent around slowly on 1 W as thats rowing and he eventually slows down so much nearly dies nolonger fights you and drag him to shore then kill him its not an easy task but if your lucky like I was a Greyling attaked it as well. I waited till he was done nearly killing my serpent and I finished him. This way you get the meat and the scales.
If I may here's a recent screeny of the fun.
https://steamcommunity.com/sharedfiles/filedetails/?id=2483038994