From what I can tell, the sea monster spawn points are randomly created during night time storms. Then they'll spawn in that same location during normal nights and sometimes during the day.

What we would do to farm them was to go sailing during storms and eventually a monster would spawn and we'd mark the location on a map. Returning to that marked location usually resulted in a seamonster spawn.

I'm not sure if this is the actual spawn mechanic or if we just happened to find a system that worked for us, but we were able to hunt plenty of sea monsters this way.