Preheat oven to 400. Line a 9x12 baking dish with tin foil and set aside.
Cut the zucchinis in half lengthwise. With a grapefruit spoon scoop out the inside pulp of the zucchini leaving about 1/4" around the sides. Place inside the prepared baking dish, brush with 1 tablespoon of olive oil, and set aside.
Heat oil in a large cast iron skillet over medium heat. Add diced onions and sauté until onions are fragrant and translucent about 5 minutes. While onions are cooking, crumble the tempeh between your fingers so that it resembles ground beef.
Add crumbled tempeh and taco seasoning to the onion, stirring to coat. Cook for another 3-5 minutes until tempeh is browned. Stir in tomato sauce and water. Let cook, stirring occasionally until water has mostly evaporated. About 3 minutes.
Spoon taco tempeh filling into the zucchini cavities so that it fills it up with a rounded top. Bake, covered, for 35 minutes. Remove tin foil and bake for an additional 10-15 minutes until zucchini is easily pierced with a fork.
While zucchini boats are baking, prepare your toppings.
Remove zucchini boats from the oven, let cool 10 minutes. Sprinkle with toppings and serve immediately.