Tuna is a very lean fish and tends to dry out quickly on the grill. While marinating and applying oil to the fish will help reduce this, if you cook tuna beyond medium rare it will be dry. To combat this on the plate you will want to serve grilled tuna with a sauce or some kind of salsa. If you are going to serve the tuna with a topping like a sauce then you don't need to do a lot to the fish. Simply brushing the tuna with some oil, seasoning light with salt and pepper will give you a great piece of fish. The topping will provide added flavor and moisture to the meal. Otherwise you can marinade the tuna in a light marinate. Anything with a lot of acid will tend to cook the fish before it hits the grill so the marinade you use should be pretty mild. Go with some olive oil, herbs, spice and a small amount of lemon juice or flavored vinegar.
If you choose a piece of tuna that is about 1-inch-thick then it will grill in about 8 to 10 minutes, flipped once. This will get through to the middle, provided that you grill is good and hot. You want to get the tuna off the grill before the surface starts to get crusty and burns. Unlike beef, lamb or pork you do not need to let tuna rest before serving. Get it off the grill and onto the plate right away.
Now for the seared tuna you want to prepare it pretty much the same way you would if you were going to grilling completely. A light brushing of oil, some salt and pepper and you are ready to go. The big difference is that you need a very hot grill. Some gas grills simply can not produce the heat to properly sear a piece of tuna. You best option in this case is to use a heavy cast iron skillet. The metal of the skillet will absorb the heat you need to get a good sear as long as you preheat the pan. If you trust your grill to produce the heat then go straight to the grate, otherwise use the skillet or a good heavy griddle.
Charcoal grills will allow you to bank up the coals, close the grate to get the kind of intense heat you need. To give you an idea of how hot you want it, the Food Network's Alton Brown sears his tuna on a charcoal chimney. This is like stacking up about ten layers of coals and setting the tuna about 2 inches from the blast furnace like heat. The reason you want this kind of heat is because you will be grilling the tuna for about 90 seconds per side. I like to get super thick cuts of tuna, like a three inch thick block and then sear it on four sides or six if you can get it cut just right. I do the six sided cube for about 45 seconds per side or a total of four and a half minutes. This gives you about a piece of tuna that has a fantastic seared crust all around and a heated middle. It's one of my absolute favorites. I've done it more times than I can count and I'm still alive.
Serve up seared tuna with a nice wasabi sauce or a balsamic reduction (boil balsamic vinegar until it gets thick, just do it outside the smell will kill rocks). Either way you prepare tuna always look for a good cut of fish. This means that the color is a deep red, but most importantly that it is even and doesn't have dark patches. Tuna is a great fish for grilling and is very unlike most any fish you have ever cooked. It is truly unique and well worth the effort, especially considering how easy it is.