BoatSpeedo is a simple to use speedometer app, optimised for sailing and motor boating.
It has a large clear display and simple controls to toggle the speed units between knots, mph and kmh.
It is GPS based and measures speed based on actual distance travelled. This differs from probe or paddle wheel based boat speedometers which measure speed through the water and hence are affected by the speed of the water itself, i.e. tides and currents.