Combine water, tempeh, and garlic in a bowl; set aside to marinate, about 20 minutes.
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, 12 to 15 minutes. Drain.
Heat olive oil in a skillet over medium heat. Strain tempeh and garlic from water and squeeze out excess water. Sprinkle tempeh with Italian seasoning. Add tempeh and garlic to the hot oil; saute until browned on all sides, 5 to 10 minutes.
Toss spaghetti, tempeh, pasta sauce, and mozzarella cheese together in a serving bowl.