Stanley Meyer's fuel cell isn't using water as fuel, it's using hydrogen as fuel, but the way it produces hydrogen (using electricity to separate water into oxygen and hydrogen) takes more power than burning the hydrogen produces (because of energy lost to heat in the process).
Because it already requires electricity anyway, the water part is pointless and it's more efficient and simpler to just run an electric motor directly.
It's kind of like if you took a shopping cart and threw rocks at it to push it along, then claimed you invented a shopping cart that uses rocks as fuel. Technically the rocks are pushing the shopping cart, but the source of energy is your arm.
I read somewhere a comment claiming that a "water engine" (Stanley Meyer's fuel cell) is possible. Wikipedia calls it a perpetual motion machine, but a quick search indicates that the guy just claimed it used water as fuel (nothing about it producing more energy than it consumed).
The thing is that the topic is very prone to conspiracy-story telling (plus the guy died young), and I don't have the will and time to separate the wheat from the chaff.
Has anyone read about it and can make a nice review of the topic?