The speedometer, together with the tachometer, the voltmeter, the hour meter, the fuel level indicator, is part of the instruments normally present on the dashboard of the boat.
The most common type is pressure, which uses the pipette to process the speed, but the measurement is not accurate, especially at low speeds.
The counter connected to a helix sensor is more accurate, especially after proper calibration. These are sometimes included in the functionality of an fishfinder.
The speedometers that measure speed using GPS are the most accurate and report the value of the SOG (Speed Over Ground). They are not widespread as a dedicated device, but are sometimes present on board NMEA repeaters that can display the speed data, coming from a GPS. Of course, the SOG data is also available on all Nautical GPS.