The water is not going to be in liquid form long enough to cause corrosive damage to valves... heck... methanol will do more damage to the engine, but unleaded gasoline and ethanol already do a number on your engine and valves, so if it stands up to that, it should be fine with alcohol injection.
This is considering a properly working water injection system, in which the water nozzles are not damaged (and damage is often done by the methanol, not the water) and the spray comes out the way it's meant to, as a pressurized mist, not as big fat water droplets. This mist flashes into vapor very, very quickly when it meets the hot intake charge, and anything that survives in mist form by the time it comes to the (hot) intake manifold and combustion chamber should evaporate when it gets there.
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