Mermaid issue?

Is there a certain time limit or exact distance at which mermaids appear? I have lost countless fights at this point because of mermaids not spawning, or only spawning once the boat is about to sink. Why? I am trying to do HG PvP with a few friends and we lost after blowing up 2 kegs on their ship at once because the mermaid only showed up AFTER our boat had sank. Can someone help me out with this?