If you are testing a new or used boat for possible purchase, don t rely on the speedometer for complete accuracy. A GPS tells you how fast you are traveling around the circumference of the earth, which is why the mph is really speed over ground. It doesn t always accurately tell you how fast you are traveling across the water. Currents, waves and navigational zigzags all affect the GPS reading. But if you are on dead calm water, there is no current and you are driving in a straight line, the GPS will give you an accurate overall reading.
Radar guns present another accurate option.
Most modern radar guns can be used from inside the boat and provide a pretty accurate reading, which is reflected from the water ahead. Older radar guns could only record a reading from a land mass or had to be held from a stationary point to radar a passing boat. The only way you can accurately radar a passing boat is to have the boat coming directly at you or away from you. Coming at you is usually more accurate because sometimes the radar gun picks up the rooster tail from a boat from the rear. If you aim the gun at a boat that is passing by you, the reading will be low because of the angle.