As a metallurgist who designed some knife steels, I can say it's not really true that cheap steel can only be sharpened so many times. Cheap steel won't take quite as sharp of an edge. It also will tend to need resharpened more quickly, tend to chip or break more easily, etc. Most filet knives will not be made out of real hard steel, as the blade needs a lot of flexibility. Meaning they will dull quickly if you use them to cut stuff you shouldn't. Luckily, fish and fish bone is relatively soft, so it shouldn't be too bad. Be careful to use wood cutting boards and avoid cutting beef or anything of that sort. They're fish specialists.
As a knife guy who likes sharp ones, I like the spyderco sharpmaker personally. It tows the line between being semi guided but also fairly quick and easy and doesn't require a work bench with clamps and the like. But it only has 2 angles. 15 and 20 degrees (30 and 40 inclusive). That's a little big for a filet knife that's usually more like 12 degrees (24 inclusive), and they are often complex bevels with 3 or more edges like a safety razor blade (12, 18, 20, and 23 degrees is a common bevel sequence). If you did a a simple straight edge, the flexibility of the blade will suffer (and is important in a filet knife), and it will dull quickly and with such a thin blade will be harder to resharpen. It can be done on a Lansky if you know what you're doing. It would take exceptional skill to be able to do such shallow and precise angles by hand on a wetstone without some sort of guide system. If you wanna do it, a system like the Lansky with a LOT of fixed, clamped angle choices would be the way to go.
But the complicated edges of a filet knife and needing to keep that flexibility in the blade, it ain't easy. This is why many filet knives are sharpened professionally, or not at all, and cheap throwaways are common in that space.
