When we were in St. Lucia, Mahi-Mahi was the "fish of the day" at virtually every restaurant we frequented. They sure do eat a LOT of it down there - lol!!
Anyway, when I make it at home, it's nearly always one of the following 2 ways (which also work for bluefish & other stronger fish, though you might not care much for them).
The first way is simply to buy a "jerk" seasoning rub - every supermarket will carry at least one brand, but taste a little before you use it since some of them are saltier than others. Sprinkle/rub onto the filet & bake at 350 until cooked through (about 20-25 minutes depending on the size/thickness of the fish). Serve with lots of lemon.
Second favorite way is "Greek Style". Layer filet with thinly sliced red onion rings, dried or fresh chopped oregano, freshly ground black pepper, paper thin slices of fresh tomato, & liberal crumbles of feta cheese. Bake at 350 until cooked through, & if necessary stick under the broiler for a few minutes to brown the cheese &/or cook/crisp the vegetables to taste. Serve with lots of lemon.