It turns out that NMFS worker Bruce Mundy went to an underwater slide show and saw a photo of silvery fish swimming in a tight circle at Hanauma Bay. He recognized the species. The fish were, um, jacks. Specifically, they were bigeye jacks, also called bigeye trevally or Caranx sexfasciatus. When I looked it up, I learned that when not feeding, these fish often congregate in tight schools.
Jacks vary in shape according to genus.
This body shape variation is my excuse for failing to name even the right family for the mystery fish, but in truth I just wasn't paying attention. And yes, Craig is gloating. Hanauma Bay was the first place I ever snorkeled in Hawaii and after 20 years of going there, it's still showing me a great time. Don't give up on the place. Marine science writer Susan Scott's Ocean Watch column appears Mondays in the Star-Bulletin.