What's an airguide Marine Speedometer?? how accurate are they?

If I bought one for my Boston Whaler, what else would i need for it to function properly?What's an airguide Marine Speedometer?? how accurate are they?
Airguide is a brand name for a dash mounted speedometer with a water pickup, probably one of the best you can buy. We would run two because old technology was what it was back then, and take the happy medium of the two and call it good. To-day we have GPS very accurate, but that only gives long and latitude distance covered by nautical miles. If you want to know the true speed on the water get a good fish-finder that has a PADDLE WHEEL on it, this will measure inch by inch better yet.

Keep-on Cruising ~~~~~~~~~~~~~~~~~.What's an airguide Marine Speedometer?? how accurate are they?
Most boat speedometers are not accurate, they measure speed by flow thru the water . GPS is probably the best way on a boat to measure speed but it measures speed across "ground" not necessarily speed thru the water. All speedometers mounted with something in the water are subject to varying amounts of turbulence and varying amounts of accuracy which may change from day to day ( just some are water temp, water conditions, salinity, etc.)