It's a "SpeedMate" (sorry, not SpeedMaster as I previously wrote) Knotmeter from Speedtech. It displays current speed, max or average speed and distance travelled. It comes in a couple of models: one where the impeller has to be stuck to your hull; and another with a trailing impeller -- basically a wire with the impeller at the end that you drag behind you. In both cases there are no holes to drill . I bought the trailing impeller, since you never know if the hull you attached the gizmo to might be out of the water. It works great, so far. (I've only had it a week.)

I bought the thing online directly from Speedtech at www.speedtech.com. They're having a small sale on it right now for $160 U.S. (down from $180).

It keeps the maximum speed even when off, until you reset it. That way your friends can't falsely claim to have beaten your record!