A watercraft immobilizing apparatus and system for a marine vessel to passively defend against an attacking engine-powered watercraft includes a towed array of entanglement lines. The entanglement lines can be provided on and administered from reusable/removable/replaceable storage cartridges mountable on a hull mount or stowed in a watertight chamber in the vessel's hull, and/or deployed via floated booms and/or paravanes to foul and immobilize propellers and engine cooling water intakes of encroaching watercraft.