Fish is one of the easiest things to cook, may people ( and I used to do this myself all the time ) over cook fish. The flesh of fish cooks at approx 80 degrees C -according to Hugh Fearnley-Whittingstall anyway, most fillets will cook with in 5 -8 mins depending on thickness. There is a tendency, especially when cooking outside to cook things until they are nearly burnt, the fear of getting poisoned etc. I'm not going to say anything on this, other than it ruins the taste of fish.
Using a preheated hot flat rock, as mentioned above sounds the ideal way to cook. Pop em skin side down, and the fish will cook lovely - the skin will be crisp and the flesh cooked all the way through - you don't need to have to rock on or near the fire while doing so either, so if you don't want as much smoke flavour on it, it's no problem.. plus if it should fall off for some reason you don't have to brush all the embers and ash off the bits you manage to salvage.