In a large skillet, bring water, wine, onion, garlic and bay leaf to a boil over a high heat. Reduce the heat to low, cover the pan, and simmer the liquid for 5 minutes. Meanwhile, cut each grape in half.
Add salmon to the liquid. Return to a boil and poach the fish, covered, until the fish is firm and it flakes easily when tested with a fork, 8 to 10 minutes. Transfer the fish to a serving platter and keep it warm. Strain the poaching liquid into a measuring cup.
In the same skillet, melt butter over moderate heat. Add flour and cook until bubbly. Whisk in 3/4 cup of the strained poaching liquid and bring it to a boil, stirring constantly. Add the grapes and half-and-half, and heat through (do not boil). Season with salt and pepper.
Spoon the sauce over the salmon. Sprinkle with walnuts and garnish with grape clusters, if desired. Serve immediately.