Cut 4 (12-inch-square pieces) of aluminum foil. Preheat the oven to 400 degrees F.
Working with 1 foil sheet at a time, place the foil sheets on the work surface. Place 1 1/2 cups of spinach in the center of each foil sheet. Top with a sea bass fillet. Sprinkle with 1 teaspoon of ginger and 1/2 teaspoon of garlic, then drizzle 2 tablespoons of Marsala, 2 teaspoons of soy sauce, and 1/2 teaspoon sesame oil over the fish and spinach. Gather the foil sheets over the fish. Fold in the foil edges and pinch tightly to seal. Place the foil packages on a heavy large baking sheet.
Bake until the spinach wilts and the fish is just cooked through, about 10 minutes. Transfer the packages to wide shallow bowls. Cool 5 minutes. Open package and fold down to reveal fish, being careful of hot steam. Squeeze the lime juice over the fish. Sprinkle the basil over and serve.