Filleting is surgery. It requires a sense of touch, even instinct, as the blade tracks tight to the rib bones and backbone and under the skin. Practice is the only way to master this skill. A dull edge is ruinous. You might be able to bluff your way with a dull knife through a duck, or even a deer, but a bad blade will botch the fine tolerances and delicate pressures of filleting a thin-skinned fish such as a speckled trout.
Most fillet knives are easy to sharpen, at least compared to the heavy blade of a hunting knife.
When sharpening, pay special attention to the point and terminal few inches. This is the portion that makes the initial, critical cuts. The blade near the hilt is used mainly for leverage. If, for whatever reasons, a particular knife resists sharpening, get rid of it or downgrade it to a lime-slicer in the kitchen. It is no ally on the tide line.