Lower unit on a boat refers to the part of the hull that is under the waterline. This area is typically where the engine and propeller are located. The term can also refer to the gearbox and other mechanical components that are housed in this part of the boat.
The lower unit provides stability and buoyancy to the boat, as well as propulsion. It is one of the most important parts of a vessel, and must be well-maintained in order to keep the boat running smoothly.