Preheat oven to 400ºF (204ºC).
Prepare the zucchini: Cut each zucchini in half lengthwise. Use a spoon or melon baller to carefully scoop out the centers, creating a boat shape.
Place zucchini cut side up in a large baking dish or on a rimmed baking sheet.
Cook the filling: In a skillet over medium heat, cook ground beef until browned. Drain excess fat if necessary. Add taco seasoning and water, stir to coat, and let simmer for 5 minutes.
Add rice: Stir the cooked rice into the taco meat mixture and cook until fully combined and warmed through.
Fill the boats: Spoon the taco beef and rice filling into the prepared zucchini boats.
Bake for 15 minutes, then remove from the oven.
Add cheese: Sprinkle the shredded cheese on top of each boat and return to the oven for 5 more minutes, or until the cheese is melted and bubbly. For extra browning, broil for 1–2 minutes.
Serve hot with your favorite toppings like avocado, salsa, sour cream, and fresh cilantro.