I wonder if you could throw a small microcontroller at the bottom of a canal, powered by water passing through a fan, to hide it. And why you would want to do that.
canals have negligible water flow (except during disasters!) so I don't think that'd work very well. They also get dredged occasionally so there's a decent chance anything you leave down there won't last more than a couple of years
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Water quality monitoring. Communicate back to the surface with ultrasonic pulses.
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