fillet knife.......

[SkunkBoat]

Dexter Russell seems to be the best reasonably priced brand(well, in relative terms). For flat fish I like a 9" straight blade. For "round" fish I like a curved blade.
I used a stone to keep sharp edge. My brother swears by the Russel diamond sharpening steel. I had one but like everything else it rusted and pitted and became useless.
Grind em back on a wheel to get rid of dings

I say this knowing I am the worst offender but, never use "the fillet knife" to cut bait, line, zip ties, plastic bottles, etc. Clean it, keep it dry. Baby it. Never leave it on the dash for a week, it will pit and you will have to grind it.