Preheat oven to 450°. In a small bowl, combine shrimp, water and lemon juice. Let stand 10 minutes.
Meanwhile, in a large skillet, heat butter over medium-high heat. Add mushrooms, onion and pepper; cook and stir 4-5 minutes or until tender. Add garlic and salt; cook 1 minute longer.
Drain shrimp, discarding liquid. Add shrimp and crab to pan; cook and stir 1-2 minutes longer or until shrimp turn pink.
Place crust on an ungreased 12-in. pizza pan or baking sheet; spread with Alfredo sauce. Spoon shrimp mixture over sauce; sprinkle with cheese. Bake 8-10 minutes or until crust is golden and cheese is melted.