I had an old tired 150 lb barnett I no longer used. I removed the stirrup. And old bow reel mount I had fit the threads. I mounted one of the larger zebcos on it upside down. Line was a lighter braided line followed by a few yards of 200-400 kevlar line on the shooting end. Cabled solid glass arrow with cable attached to an altered speargun tip that fit over the end of the arrow and allow the arrow to fall free when shooting a fish/gator. Line was attached to a sliding marine swivel on the cable. Had to sit down and put my feet in the limbs to pull it back. Had intended to use it Alligator hunting, but never did. Shot fish for jugline bait on a few occasions while wading river pools. With the heavy arrow and line it was not that much more powerful than a compound bow, plus I think I kept the limb mounting bolts backed out a turn or two when I adjusted the tiller. The first year crossbows became legal for hunting I temporarily turned it back into a hunting crossbow. It was stolen out of the back of my truck. I did it only because it was convenient. I would not buy anything to build one on purpose and have no plans to make another. For an extra power bowfishing rig, a compound bow is much more convenient and just as effective. One of the newer more powerful/faster crossbows might not be safe for that rig either.
"Onward up many a frightening creek, though your arms may get sore and your sneakers may leak. Oh! The places you'll go!" ~ Dr. Seuss