Combine first 3 ingredients in a medium bowl; stir well with a wire whisk. Add shrimp; stir well. Cover and marinate in refrigerator 15 minutes.
Drain pineapple, reserving 2 tablespoons juice. Combine reserved pineapple juice, water, and next 7 ingredients (water through 1/4 teaspoon salt); stir until well-blended. Set aside.
Remove shrimp from bowl; discard marinade. Heat 1 tablespoon oil in a wok or large nonstick skillet over medium-high heat. Add shrimp; stir-fry 1 minute. Add onion and mushrooms; stir-fry 2 minutes. Add ketchup mixture and peas; stir-fry 1 minute or until thick and bubbly. Remove from heat; stir in pineapple. Serve over rice.