Halve 1 of the garlic cloves and reserve. Finely chop the remaining cloves. Heat 2 tablespoons of the olive oil in a large, heavy-bottom skillet. Add the chopped garlic and onion and cook over low heat, stirring occasionally, for 5 minutes, or until softened.
Stir in the beans and tomatoes and season to taste with salt and pepper. Cook gently for an additional 5 minutes.
Meanwhile, toast the bread on both sides, then rub each slice with the cut sides of the reserved garlic and drizzle with the remaining oil.
Stir the shrimp into the bean mixture and heat through gently for 2-3 minutes. Pile the bean-and-shrimp mixture onto the toasts and serve immediately, garnished with watercress.