This is only useable when the gates are nearly shut off, otherwise too much water flow. I've snorkeled this many a time and it can change dramatically month to month. I've seen uncountable schools of 2-4lb largemouth, 30lb musky, white suckers, and carp. Small sunfish and oddball small fish. The bottom content close to the dam is sand and rock that out farther transitions into mud. Please note that local regulations regarding public use is under constant attack, and a number of special interest groups are constantly attempting to remove and restrict all public access to this lake, so recheck the local regulations before using this site. Finally I would mention that the current can be fastest close to the dam, it drops down on the edges and farther down stream, with just snorkeling gear it is possible to walk out farther downstream or on either side farther away from the dam. The density of fish population in this site is directly proportional to the amount of water coming out during the spring floods. If the dam was open in the spring then the creek will be full with fish. If it was shut during dry low water almost zero fish will be here as this creek freezes solid during winter (all the deeper overwintering holes have filled in with road sand). Vis is roughly 4-6ft in the summer. But close to the cities and a different experience. I've done this many times with snorkeling gear and my kids love it.
Gray Bay Dam is the head waters of the Minnehaha creek that flows through the twin cities. The area just down stream of the dam has public access for canoe launching.