You're right - I recall reading about such a system a few years back - but can't remember who offered it. Haven't seen anything about it lately so maybe it never caught on.
The main drawback may have been the need to lead a wire down the keel to one of the transducers. (in one version that I remember)
I think it used the doppler effect or transit-time ultrasonic technique, a common industrial flow measurement method. As a flow measurement it is a bit unreliable and subject to errors that may be why it hasn't caught on here.
Makes sense, though, to eliminate the moving parts, doesn't it?