I used to use a Dexter Russell High-Carbon wide stiff boning knife to fillet fish when I worked on party/charter boats in Gloucester. My preference is the 1376, with a six inch blade. The commercial groundfishermen use 1375, five inch "rippers" to gut fish, and a lot of the party boat crews use the 1377 and 1378, seven and eight inch boning knives respectively, to cut fish. Properly maintained, these knives take a razor edge and a light oil (I know a couple of gillnetters who used to put them in water bottles with the tops cut off full of olive oil) will keep them from rusting. They are easily sharpened on a stone and the edge can be maintained with a butchers steel. I've sharpened mine before I began cutting and touched it up as needed on a Dexter-Russell sharpening steel. We used to cut a large volume of fish, upwards of 500 cod, haddock, cusk, and ocean catfish (wolffish) per trip, and when fishing was good for them, over 700 bluefish in a night. I still use it on the occasional codfish or ocean catfish I take home for dinner off the lobster boat, and haddock, striper, flounder and bluefish I catch recreationally. I've cut up porbeagle and blue shark with them and they stayed sharp, even with the tough skin of a shark. They are inexpensive so you can buy a few to keep on the boat, and if you lose one overboard, no big loss.
Pete