People often ask me how to grill fish properly. My first rule is don't fret, fish is pretty easy to be grilled. Direct heat cooks fish fast, evenly and without removing moisture which is why grilled fish is quite flavorful and juicy. My second rule is make sure it doesn't stick. Whether you oil the cooking surface or brush the fish with a little oil, make sure that you have a non-stick surface to work with!
Apart from that, it is easy and fast to grill fish. One of the quickest foods to cook, fish can be on and off the grill in a matter of minutes, a perfect after work meal. Pick up a couple of fillets and lemon or two and light up the grill as soon as you get home. Marinade or season the fish as desired.
Then grill it!
The hardest part is knowing when it's ready. When fish is cooked the meat will flake easily with a fork and will appear opaque all the way through. If any part of the meat is still glossy and partially translucent then it's not done. Do not serve undercooked fish, it is not safe and may turn someone off fish for life.
To make it easier, always start out with a steak or fillet that is evenly cut. If one part is thicker than another it will be hard getting the thick part cooked before the other part dries out. Consider cutting your fillet in two if it's uneven. Put the thick part on first and when it's about halfway done, put the thinner part on. This way you will have properly cooked fish without burning anything.
Most fish is sold whole or in fillets or steaks. Fillets will tend to fall apart a little more easily. This is where my rules come in. With an oiled surface, put the fish on the grill and leave it until ready to flip. Flip gently and leave until it is ready to be taken off the grill. With fillets, you can tell it is ready to flip because the edges are flaky and opaque. Do not attempt to grill large fillets, but instead, cut them into smaller pieces that will be easier to handle. Steaks and whole fish hold together better but take longer to grill. If you are grilling whole fish stuff it with something like lemon slices and herbs. This not only adds to the flavor but creates a space inside the fish to let the heat cook through more evenly.
Also, keep some fresh lemon juice and maybe some melted butter handy while you are grilling. You can brush this on as you grill to add flavor and keep the fish moist. But remember that butter will burn so be careful with it. I love dripping lemon juice over fish while I grill it. The steam and the sizzle add to the show and makes everyone appreciate the meal just a little bit more.