For a conventional sailboat, a different arrangement can be used.
In the project that I am considering, 2 transducers will be able to measure the component of the speed of the water parallel to the axis of the ultrasonic beam at a distance of 30 cm from the hull.
By knowing the installation and the heel angles, it is then possible to calculate the lateral speed of the boat.
A good introduction to the pulse-to-pulse coherent Doppler technique can be found in these documents:
“Pure coherent Doppler systems – how far can we push it?” Lohrman and Nylund, 2008.
“Performance of a Single-Beam Pulse-to-Pulse Coherent Doppler Profiler” Zedel, Hay, Cabrera and Lohrmann, 1996.