To make the marinade, place 1/2 cup olive oil and 1 sprig of rosemary into a resealable plastic bag. Add the marlin steaks, seal the bag, and turn gently to evenly coat the fish. Refrigerate for 1 hour.
Preheat oven to 450 degrees F (230 degrees C).
Pour 1/4 cup olive oil into an oven-safe 10 inch skillet. Stir in the onion, cherry tomatoes, corn kernels, garlic, 1 sprig of rosemary, and kosher salt.
Place skillet in preheated oven, and cook until vegetables are soft, 20 to 30 minutes.
Preheat an outdoor grill for high heat. Lightly oil grate, and set 4 inches from the heat.
Place the fish on the preheated grill, and season with salt and pepper to taste. Cook, turning once, until fish is opaque and flakes easily with a fork, about 5 minutes each side depending on thickness. Place the fish on a serving platter, and top with corn and tomato relish