Ditto Cheryl and a few other points.
You alluded to protrusions, never a good thing in a koi pond. Also, it doesn't take very long for the lenses to become coated in algae...another maintenance item. I have even had pool lights in a koi pond and although protrusions were not an issue, keeping the lense clean was.
Like Cheryl mentioned, overhead lighting is preferrable to me, afterall, koi are bred to be viewed from the top anyway.
Steve