Melt the butter in skillet over medium heat. Add minced celery, onion, and red bell pepper. Cook for 2 minutes, until tender, then cool. Remove the stems from mushrooms and finely chop 3/4 cup of stems. Set the mushroom caps aside. In a large bowl combine vegetable mixture, chopped stems, crushed oyster crackers, lobster meat, crab meat, 1/4 cup shredded white cheddar cheese, egg, water, Old Bay Seasoning, garlic powder, salt, and pepper. Grease two large jelly-roll pans. Brush mushroom caps with olive oil, sprinkle with salt, and stuff with lobster-crab mixture. Arrange caps in prepared pans. Top with remaining 2/3 cup shredded white cheddar cheese. Bake 12 minutes - until lightly browned. Makes about 50 mushrooms.