1. Preheat the oven to 400 degrees.
2. Place the red snapper fillets in a baking pan at just large enough to hold the fish snugly in one layer.
3. Spread the onion slices along the top of the fish, and drizzle with the 2 tablespoons of olive oil.
4. Add the wine, water and lemon juice.
5. Season to taste with the salt and pepper.
6. Place pan in oven and bake until fish is done, approximately 20 minutes.
7. Transfer fish to serving plates, spoon some of the liquid from the pan on top and serve.