A sailing buddy spotted this fin at a local surf spot a few days ago. this thing is for real...the "turbo tunnel" fin. Used on surf boards and some SUP boards. I remember a fin in the early 90's that used much smaller tunnels (or channels) to address cavitation. This is supposed to be about generating lift and drive, though

www.peconicpuffin.com/the_peconic_puffin/2010/07/strangest-fin-seen-in-25-yearscould-it-work.html

Certainly it looks more functional than the old "football fins" though probably a weed catcher.