Preheat oven to 400F degrees.
Cut spaghetti squash in half, lengthwise. Scoop out seeds and strings from the middle. In a large glass baking pan, fill with about 1/2 inch of water. Place spaghetti squash in baking dish, skin side up. Bake for about 45 minutes. Set aside.
Heat 1/2 T in two medium skillets over medium heat. In one skillet, add garlic. Simmer for a few minutes until garlic becomes fragrant. Then add shrimp. In the other skillet, place zucchini. Allow both skillets to cook, stirring as needed, for about 5 minutes. Then add mushrooms to the zucchini skillet. Allow both skillets to continue to cook for another 5 minutes, again stirring as needed.
While food is cooking on the stove, lift spaghetti squash from the baking dish. (Note: Squash may still be very hot. You may want to put on oven mits to handle the squash.) With a fork, scrape inside of squash to create string-like spaghetti into a large bowl.
When shrimp and vegetables are cooked through, remove from heat.
In four serving bowls, evenly divide spaghetti squash. Top with shrimp and vegetables. Lastly, pour marinara sauce on each dish. Enjoy meal hot!