re: water car
The best advice IS to do the math, make sure the thing can be independantly verified before believing it. Do lots of homework and don't rely on anecdotal evidence. I shudder to think what sort of "proof" you will be presenting if you cannot be exacting in the process.
But to most people it is a bunch of hooey, guy would not let anyone measure the input and output power (citing patent crap) thus preventing anyone else from validating (or invalidating) his claims, it smells like a sophomoric scam to me.
If you wan't to spend YOUR time to chase after it, go for it, but don't be mad at the experienced sceptics for calling it like they see it. And expect to have some serious details sorted out before anyone will believe you, not the crap thats online.