Think of it in terms of physical effort.
To break the bonds physically takes a lot of electricity, brute force so to speak.
No way your going to get more than you put in, as a matter of fact its pretty inefficient.
Now instead of smacking those bonds with brute force, You use a scalpel of sound waves to sever them. Power required could be little more than whats needed to run a stereo. Its all about the Frequency, NOT about the brute force.
Like shattering a glass with sound. If you have the right frequency for the glass, it doesn't even have to be loud. You simply resonate it till it breaks itself apart.
Same for H20, and its bonds, find the right frequency in the right chamber or sound reflective setting and it could shake itself apart. Freeing large amounts of H and O2 to be collected and burned.
This IS the answer for the future, and we will solve it, its just a question of time.