Heat oven to 400°. Cut 2 pieces of 16-inch-long parchment paper. Fold each in half lengthwise and unfold. Place fish along one side of the fold.
Combine garlic, ginger, turmeric, and oil in a small bowl. Season with salt and pepper. Rub fish with garlic mixture. Squeeze lemon over top, sprinkle with coconut water, and dot with butter.
Fold parchment over fish leaving space for steam to circulate. Seal by making small, overlapping folds around edges. Transfer to a baking sheet and bake until fish is cooked through, 10–12 minutes depending on thickness of fillet. Remove parchment and serve.