This filleting knife does not rust, does not dull easily, is easy to sharpen, and the handle is sanitary, comfortable, and good in cold conditions. Most importantly, the blade is flexible, thin and the shape is just right for filleting.
I’ve used mine for about eight years. It came razor-sharp from the factory and stays sharp for a good deal of time. These days, I usually sharpen it a little bit before every use. A couple laps on a 220-grit Japanese waterstone does the trick.
There are plenty of fancy fillet knives you can get. This one is not particularly expensive, and it’s the brand I see most commercial fishermen using. There’s also a plastic scabbard you can buy. Dexter’s filleting knives come in a few varieties of size/length, etc. There’s the 9-inch narrow one, for instance. Personally, I find that one a bit bulky, so I use an 8-inch narrow.