>>There's a lot of everything in seawater, including gold. The problem is getting it out. Very low concentration, thus very high processing costs.
Hydrogen is fairly prevalent in water on the other hand :)
As for releasing nasty gasses you could just dissolve them straight back in, without noticeably affecting the concentrations. I'd rather suspect even in the immediate vicinity, the differences would be very small though I don't have a clue how many mega-litres of water would be processed per day.