Howdy,
With the roots style positive displacement supercharger, the booste is totally dependent on the engine rpm so even at a cruise the egine will see booste per se as the blower crams a fixed amount of air for every rotation into the intake. As far as injecting water into the engine, its a very well known fact that cooler engines make more power and are esier on parts so reducing operating temperature will improve the efficiency of the engine by increasing the fuel air charge density and cool the intake tract. The problem with the vacuum idea is that roots supercharged engines make almost no vacuum because the blower is always forcing more air in and the manifold vacuum is offset by the forced air. This problem is even worse at cruise because while the engine is making more vacuum the blower is moving more air. To inject water at cruise would require some sort of pump to actively feed a metered water amount. I don't know what the long term reliablity of a windshield washer pump would be but it would be a likely choice because it runs on 12V DC and is designed for water.
Just the crazy ramblings of a Physicist with a passion for cars,