I have spent 40 years searching for the perfect fillet knife. Typically I fillet 40-50 5lb+ fish at a time several times a month.
The problem is not just corrosion but to get as much meat as possible it is necessary to run the blade along the bones and with large quantities the blade deforms. Folks who believe a "touch up" with a steel every few fish like you do with meat will work are mis-informed. Steels only align edges there dont sharpen or fix nicks from bones........
You need a hard, semi-stainless blade period. Enclosed is a photo of a few of my knives, victorinox and green river are the best medium priced knives and the likes of bubba, and rapala etc are just rubbish. Their edges deform rapidly and hardness is inadequate;
I tried a few top end Japanese knives for a while including Shun which were excellent but just too thin and fragile for when you cant be bothered reaching for a chopper and take a head off.........the answer to your question in my opinion is the german Wustof 'trident' range, I use their 6 inch fillet, only fault is at 6 inches its too short BUT I can fillet 100 fish without touching it up;