As far as i know, world of Valheim is split up into zones, each 64x64 meters. Every time you enter a zone Sea serpent has 5% chance to be spawned. Im talking only during the night or storm. So your best way to find a Sea serpent would be to sail around at night. Throwing food around will not help, trust me i tried :)
They dont eat food, they just kill animals and leave food. However, they do love snacks.
Here, have a look at my Sea serpent pet having a snack :)
https://steamcommunity.com/sharedfiles/filedetails/?id=2741002642