I don't believe that would happen (ackermann, i think the very spark you're using to electrolyze the water may in turn ignite the hydrogen and oxygen gas mixture causing a catastrophic boom. or, if contained, it would at least be a lot of heat remaining.) because the gasses does not necessary...
well, maybe one could take a huge electricaly isolated tank of salty water (not very expencive), place a kind of lid that is able to move without much problem (like a piston), place the tank high, place a conducting material (cooper rod) in contact with the water which will be the “intake” from...