I was actually snorkeling in Horseshoe Bay when I encountered them. I swam from shore out to the small island that is about 300-400 metres off shore, and on my way back I encountered them along one of the underwater cliffs on the north side of the bay. I started following them and managed to stay with them for perhaps five minutes. They weren't moving all that fast.
Matt added that the fish were about eight inches long. The shape, colouration, behavior and presence of an adipose fin are characteristic of the whitefish/cisco/lake herring subfamily (coregoninae), of which a handful of species are known from the waters off Pukaskwa. Without a specimen in hand, my best guess is that these are juvenile Lake Herring, also called cisco, (Coregonus artedi).
Water temperatures in Lake Superior are very high this summer.
Thanks for sharing the great photos Matt.