The hydroelectric generator system can include a structure supporting a plurality of watermill units and having an enclosed internal volume and a plurality of lower openings, a stabilizer reservoir having an internal volume and positionable above a center of gravity of the system and an aperture, and connection tubes providing internal fluid flow communication between the stabilizer reservoir and the internal volume of the structure; wherein air can be extracted from the internal volume of the structure via the stabilizer reservoir, and blown into the internal volume of the structure via the stabilizer reservoir, to lower or raise the hydroelectric generator system in water, respectively.