For the rice: Combine the rice and water together in a large saucepan. Place the pan over medium heat and bring to a boil. Reduce the heat to low and cover the pot. Cook the rice for 20 minutes.
For the sauce: Peel the mango and slice the flesh off the pit. Dice the mango flesh.
In a large bowl, whisk together the vinegar, oil, and lime juice. Season with salt and pepper. Stir in the mango, cilantro, garlic, and ginger. Set aside.
Heat a large, nonstick saute pan over medium-high heat.
Rub the fish fillets with the oil, salt, and pepper. Place in the hot pan and sear until golden on 1 side, 3 to 4 minutes.
Turn the fish over and cook until cooked through, another 3 to 4 minutes. Be careful not to overcook the fish. You'll know the fish is done when the flesh springs back. Remove the fish to a plate.
When the rice is cooked, fluff it with a fork and gently stir in the cilantro and scallions. Season with salt, to taste.
Place a mound of rice on each plate. Top with mahi-mahi. Spoon the mango sauce over each fillet and top with chopped scallions. Drizzle the liquid from the mango sauce around the rice.