Like has been stated, they have spawn points in the oceans biome.

On a related issue, I have just started playing again and I know the serprent behavior has been altered, is it just me or are they harder to kill ashore now?

It used to be you just pulled them ashore and boped them, now they seem to wriggle back to the water alot faster.
And while they dont run away anymore they seem to aggro on NPC mobs and chase off after them, I have had to run along the shoreline barelly keeping up with serprents who keep targeting and swimming for NPC's further away.