Water does not add horsepower. Combustion does not break H2O down into 1) H and 2) O's, it creates steam! ADI was, and still is injected into high performance or high altitude engines to prevent detonation by lowering the cylinder head temperatures. Water mixed with alcohol is injected in and on the engine to cool it. Look at any of the Reno recers up close for a detail.
As for the old fuel, I'd run it out on the test bench or dump it into your car's gas tank. Your car would see little difference between our alcohol based fuels and commercial Dry-Gas or other additives.

Chris