When we do mahi mahi, I usually go simple flavor: fresh lemon juice, minced garlic, butter, salt, pepper, dash of sugar, dash of cayenne pepper.
I will melt the butter to where its hot, and put the crushed garlic and pepper in there to let the flavors soak into the garlic, since they are fat soluble.
Heat the grill as hot as you can, and place a sheet of foil over the grates to super-heat the grates. soak a paper towel in vegetable oil and rub that on the hot grates several times (It can ignite, be careful. I get the grates hot enough the oil can ignite right on the grates). About 3-4 minutes a side, flipped once, and its good to go after a few minute rest.