Bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally until cooked through but firm to the bite, 9 to 11 minutes. Drain and run under cold water.
Mix tomatoes, onion, cucumber, celery, crabmeat, and tuna in a bowl; add spaghetti. Stir in salad dressing and garlic; season with salt and black pepper. Cover bowl with plastic wrap and refrigerate for flavors to blend, at least 30 minutes.