http://airmartechnology.com/2009/marine/product-search-results.asp

Uses high frequency pulses to detect and measure small particles in moving water to give the speed.

Would love to try this myself, as my paddle wheel sensor is constantly getting gummed up by these almost clear gelatinous worms that seem to secrete some sort of calcium trail.

But my instruments are older than the NEMA standard... would have to change everything out.