You have to imagine how large koi get. They easily reach two feet in length.
I've seen them in a foot of water, but I felt very sad for the fish. Imagine
you are 6 feet tall and live in a room 6'-2" high.

Point taken.

Koi was just the first thing to come to mind and then of course there
was the huge Koi pond in our Hotel over the Easter break that got me
thinking..

Goldfish on the other hand never had it better ;-)

Any other cold water fish that would fair well in that depth pond?