It's not converting, it's not linear. It's like being a bank. You loan cash, and get it back (the water capacitor never discharges from the cathode to the anode, it charges, does not reach critical voltage, and runs back down into your battery). You could do it with an AA, but only a rechargeable one. and it would wear before it's done the whole ocean, and lose it's ability to be recharged. But alas, ocean water is not dielectric. So it's conducive to current flow.
The whole HHO buzz is not about HHO, it's about how we get it. And for how little. HHO isn't even HHO. It's Hydrogen and Oxygen, a lot of it monatomic.
I didn't say zero, I said practically zero. There will always be a loss of energy, however small. But, take note, I'm not talking about splitting the water directly, but rather obliquely. the point is to not run current through the water, but instead charge the plates and uncharge them several thousand times a second. through doing this the water is split, but indirectly. Ie, you're using a high voltage CHARGE on the water capacitor, and letting it dissapate back into the battery on the off cycle.
I find it is the cult that shun's logic and foreign ideas. I believe the "Oh duh, we know better" attitude has been adopted by every cult in history. So maybe every HHO thread has one or two non-occultists, which you find surprising and I find refreshing.