Cut the fish into strips before marinading. This allows more surface area and more marinade to be absorbed. It also helps to cook the fish more evenly. Large pieces of fish IMO tend to dry out on the outside and still be moist inside. I prefer smoked fish to be fairly dry throughout.
Be creative to make a marinade.
For kingfish I will use Milk, Water, Lea & Perrins, Hot Sauce, Dales Seasoning, Lemon Juice, and some random spices.
Cook at a low temp.
I caught a 35lb. Kingfish last weekend. I already cooked a third of my share but plan on smoking the rest.