No, it's a scam.
(You can use a lot of electricity to break up the water into hydrogen and oxygen - https://youtu.be/d85OX6yEwE0 - and then get the energy back by burning the hydrogen, recreating the water. But hydrogen has big disadvantages as a fuel for cars.)
Its not a scam. Well maybe some of the "kit" vendors, but not the conversion in general. I personally know someone who did this.
The conversion isn't a replacement. Its more of an additive that makes your fuel burn cleaner and last longer because now some of the fuel is replaced by HHO. Your alternator generates plenty of juice to keep the system running. And it can be done on any gas/diesel car regardless of make/model/year.
This guy explains it pretty good https://youtu.be/WMiVdxBToGI
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