Faria Kronos 4" Speedo 60 mph with Stainless steel trim. Electronic speedometers operate by capturing pulse produced by a sensor.
The pulses are then electronically converted to a speed reading like a tachometer converts ignition pulse to RPM.
Pitot tube type speedometers operate by pressure from the water being forced into the pitot tube.
This pressure is then transmitted through flexible tubing into the bourdon tube movement inside the speedometer head where it is converted into a speed reading by the movement mechanism.
Pitot sender kits can be found here