I doubt it will have as much power if you're just putting water into the gas tank, which means it will stall when maxed out. Now there are some cool things you can do with water that will yield some efficiency and power gains. For example, if you inject atomized water into the intake air it will lower the temperature of the air and increase its density, producing more power. This dense, moist air will also produce less of what they call detonation, so more timing advance can be added, equating to more power if you tune for it. These qualities are especially beneficial in turbo and supercharged engines that are prone to these problems.

Problem is, small engines run off of carburetors and magnetos, so tuning is a bit tricky compared to a computer controlled car engine, and you certainly won't see any of these gains by just dumping water in the tank and leaving it at that. I say again, you will lose power in your generator and when you hook all your mining machines up to it and run them at full tilt, my guess is the engine will stall out.

Something you might want to look into however is getting either a natural gas or propane conversion for your generator and running off of those. I think those fuels are still much cheaper than gasoline or diesel, and both are delivered to your house (propane if you do a whole house setup, and nat gas if you live in the city.)