I saw a kayak that had a “fin” on the bottom, about 4" in depth and 18" long. The kayak appeared to be made out of kevlar, as the guy carried it easily to launch. About 17’ long. I wanted to ask him about it, but he was in a hurry to get paddling. What kayaks have such a fin, or was this custom made?
Thanks.
Sounds like
a “hanging skeg” (similar to hanging chads) to me 
guess
Old Nordkapp or Icefloe?
My inflatable
Innova yak, by Sunny, has a movable aluminum skeg, it’s designed to install at one end or the other when setting up for single or dual paddling.
Being an inflatable, maybe I’m comparing apples to oranges.
most likely a racing kayak
a bunch of them have an under stern rudder system for more positive turning…
Shearwater
The Heritage Shearwaters and possibly some other kayaks have a fin box that accepts surf board skegs. The skegs are usually too big and cause too much drag. I made my own skeg for my Shearwater and it is fairly small. Just enough to offset the wind but not inhibit turning too much.
Kirton Tercel
has a shallow rudder off a 2 1/2" deep skeg