As someone who currently has 4 Serpent trophies hanging in his base I can tell you this: If you go looking for them they will never come XD
I found all of them randomly while doing other stuff. You don't really need them for anything other than Serpent Stew so my advice is do other stuff and let them surprise you.
I know it's not very helpful but that's how it is.

P.S. They usually spawn pretty close to land, because you need to drag them to shore. I.e. if you are sailing in the middle of the ocean they won't appear.