Interesting, but it's not "salt water": https://www.hotcars.com/this-48volt-quant-gets-600-miles-on-a-tank-of-salt-water/

"bi-ION is a salty aqueous electrolyte solution. It is not plain salt water! The bi-ION electrolyte is made of specially nano-structured molecules, which are the energy carriers and an aqueous solution with dissolved salts to make the electrolyte pumpable."

Admittedly I don't know much about this, but this is something else entirely. Interesting though.

* It seems like this is a derivative of a salt battery. The power is not coming from the water, but what is in it, specifically the catholyte that is producing electricity. This is super interesting, but given how no one has ever heard of it, I'm going to wager there is some limiting factor or cost. Data is sparse on this car/engine.

There it is..... https://qz.com/261450/this-sports-car-runs-on-saltwater-but-its-no-threat-to-tesla/

$1.7 million

and (from the original link)...

The technology however is not perfect, facing several significant challenges including limited power. Therefore small supercapacitors and/or Li-ion batteries are still required for acceleration and regenerative braking which might be why, after 6 years NanoFlowcell has yet to go into production with any of its vehicles.

This is factually correct. The problem is extracting hydrogen from H2O is very energy intensive, especially through electrolysis. Also raw H2 is very explosive, as is O2, making that progress quite dangerous at even moderate scale. The hydrogen in hydrogen cell vehicles does not come from water.