A motor that's mounted entirely inside a boat's hull.

a boat motor placed inside the boat

includes any motor located within the confines of the boat

An engine mounted in the watercraft, with a propeller shaft protruding through the bottom of the boat directly from the engine.

An inboard motor is a marine propulsion system for boats. As opposed to an outboard motor where an engine is mounted outside of the hull of the craft, an inboard motor is an engine enclosed within the hull of the boat, usually connected to a propulsion screw by a driveshaft.