Compasses are devices for determining horizontal direction for safe and accurate navigation over land, in the air and on water. There are several types of compass, each for its own purpose. You may want to re-visit the descriptions given in 'Compasses and Bearings' . Below is some additional information about compasses used for marine navigation.
Marine compasses are usually liquid-filled.
The compass card is mounted above the magnetic needle mechanism. It is finely balanced on a needle point, floating and able to swing around to align with magnetic north. The reading is taken from the fixed lubber line on the body of the compass. The lubber line is pointed in the direction of the head of the vessel for steering on a given bearing. An example of a marine compass is shown in cross-section below.