You are taking about hydrokinetic turbines. Aka run of the river. These also work in ocean currents and tidal currents as well. The Gulf Stream has multiple hundreds of terrawatt hours flowing past it just the Florida portion of the current has 163TWh/y up for grabs. River in the continental USA 120TWh according to the USGS and DOE studies. Marine energy with waves and tides is 2,300 TWh again from DOE. Add in ocean thermal power and that adds.4000+ TWh more. Of course solar dwarfs any of those by a factor of a million times. The earth receives well over a million times the entire human energy use per day every day. 3% of the area of the Sahara desert can power all 8 billion of use at EU levels forever. It’s has never been, never will be a energy source problem it is a energy storage , and transport problem both of which have technical solutions.
that’s multiple hundreds of nuclear plants and these sources are 24/7 currents or 6-8 on switch flow with slack tides of a couple hours with tides being predictable years in advance.
https://www.sciencedirect.com/topics/engineering/hydrokinetic-turbine
https://www.energy.gov/eere/water/articles/doe-funded-study-pinpoints-potential-lies-within-waves
Yes Hydrokinetics is indeed true free energy if we can build the mechanism to harvest it unlike Windmills that don’t last long enough to pay for themselves. the obvious benefit right off the bat is we dont need thousands of pounds per structure footings and the difficulty in servicing something 100s of feet in the air...