I used to fish the Gulf of Mexico for snapper and grouper and have cleaned countless fish from mullet to sharks (cutting shark-skin will dull a knife in nothing flat!). My favorite two knives were the Normark
Presentation Fillet Knife and the Normark Presentation Hunting Knife (the hunting knife is no longer in production unfortunately).
The fillet knife has a 6" blade, 1/16" thick and 3/4" wide at the hilt. It tapers nicely to a delicate tip and is great for skinning and filleting. My complaints with it were that the handle material (some kind of molded plastic) became very slick with fish blood and slime (the new model has a soft grip) and since the blade was rather delicate, cutting through heavy rib cages could only be accomplished right next to the hilt.
The hunting knife has a 4 3/4" blade, 3/32" thick and 3/4" wide at the hilt which is the same handle as the fillet knife. This knife retains its blade thickness and width to withen the last 1 1/4" of the blade. Since it is a more robust knife I used it far more frequently than the fillet knife, so I actually carried the hunting knife and kept the fillet knife handy for more skilled cleaning.
I think a design that incorporates the best of these two knives would be a great knife for fishing. A 3/32" thick blade, 3/4" wide at the hilt and an overall blade length of around 5" or 6" that tapers more in width and thickness than the hunting knife to give it better flexibillity and a handle material that wouldn't become so slick (material unknown, ideas anyone?), oh, and a stainless blade material that wouldn't need sharpening so much (my knives are nearly 25 years old and I don't know what stainless they were using then but sharpening took place several times in a cleaning session!-Guy T.