What if the rats (centuries of maritime experience in their furry little heads) have turned off the signaling device, rendering them 'invisible', and have gone rogue, or 'piRAT'?
A boat with a working engines comes into view, rats lure them closer, and board her?
Rats, leaping through the air, swinging on ropes, etc. Kill and eat the crew, if the second boat will hold all the rats, take it over, if not, use it to tow their ship near to isolated, fertile land.
Bold adventurers, brave little heroes, and born survivors.
It's really only a horror story for the humans involved, and the weaker rats.