If you fry the fish - like many have said, you won't get a noticeable fish taste. however, if you bake the fish in cast iron (with the fish resting on the metal bottom), you will get more of a fish taste in the iron pores which you will notice in cooking lighter tasting foods (like many vegis and chicken).
If you get the fish taste in the pores, the solution is to put the skillet upside down over a fire and burn it out and re-season it.
Cast iron is GREAT for fish. It is the only kind of skillet I will use for blackened fish. - so simple and so moist and tender...
Heat your cast iron skillet till it is at 450-500 (a drop of water will dance and spit). Brush a layer of butter on the fish, sprinkle your favorite seasoning, then drop the fillets in 2 minutes or so per side (if using thick fillets - adjust the time till the fish forks apart easily). Eat them while hot - nothing better! The high heat sears the outside and keeps the inside moist.