If you have less than 75% Sanity at night, Wavey Jones can spawn and attack boats near you, even if there is no one with less than 75% sanity on them.
Meaning it's dangerous to so much as pass by your parked boat under those conditions, because you might unwittingly unleash Wavey Jones upon it and end up seeing it sail away... or worse, sink.
1. Have less than... whatever the sanity threshold for Wavey Jones is.
2. Sit near, but not ON, a boat.
3. Wait for Night.