Speaking of salt water, the lagoon we have on campus is ridiculously salty - the dirt path next to it is so imbued with salt that the salt crystals ended up acting like cement and forming a super-strong surface XD
Surprisingly it still has a lot of life (and is /very/ green) - (They pump in fresh sea water, and pump out some of the lagoon water to prevent it from getting too stagnant.)
But I don't think that a salt water powered car would be all that efficient.