...we could assume this process somehow alters the molecular structure of water. IOW, the usual H2O molecule (actually, a solution of H+ and OH- ions) is somehow rearranged into a new collection of these "molecular clusters".

But we're still talking about hydrogen and oxygen, there is no other element mentioned here that could be used for energy by combustion. And when you burn hydrogen in oxygen, you get back the exact same energy you expended to rearrange the original water.

Actually, there is an industrial process that does this. Using lots of electrical energy and catalysts you can convert water into hydrogen peroxide (H2O2) which is highly unstable and quickly breaks down into water and free oxygen, yielding a great deal of heat. The NAZIs actually experimented with this process during the war and used it to power submarines with performance that rivaled some of our modern nuke boats! Not only could these subs achieve 25 knot speeds submerged, the by products of the recombination reaction were simply steam and oxygen, the former drove the turbines, and the latter the crew could breath.

Fortunately for the Allies, like so many other NAZI wonder weapons, they were not available in sufficient numbers to make a difference. They were extremely expensive, dangerous to operate, and prone to breakdowns, but they were deadly. And it took a LOT of energy to produce the concentrated H2O2 they used for fuel.

https://en.wikipedia.org/wiki/Type_XVII_submarine