(1) The rocking-horse motion of a vessel in the sea. (please see also: Roll, Yaw, Heave, and Scend)
Pitch (2) Of a propeller, the distance that a blade would move forward in one revolution if it were cutting through a solid. In practice there is a certain amount of ‘slip’. In choosing a propeller it is important to get correct pitch and diameter. These dimensions are related to the power of the engine, the rate (RPM) at which the propeller turns, and the speed of the boat. The short answer is to get technical advice, usually from the propeller manufacturer.