A sensor to measure velocity underwater

Can anyone point me to a sensor that would allow me to measure velocity underwater?

Thanks

Low speed? Get a “paddlewheel” type speed transducer from a local or online boating / yachting supply store (chandlery) or make your own. Little paddle wheel sits half in, half out of a cup (which is embedded in hull of boat). As boat moves, paddlewheel turns. Affixed to the paddlewheel is a magnet which trips a reed switch or hall effect sensor each revolution sending you a pulse.

Just to clarify, do you mean the speed relative to the water or relative to the earth as the object travels through water?

i would be measuring the speed relative to the water, assuming the body of water is still (lake/pond) and i am moving an object through it. The speed would be low, and i’ve seen those paddle wheel type implementations, but would it be possible to build something that would work w/the arduino? If so, how would you build it?

thanks for any input