Preheat your oven to 400 degrees Fahrenheit. Lightly grease a baking dish.
Cut the zucchini in half lengthwise. Use a spoon to scoop out the flesh, leaving about a 1/4 inch thick shell. Chop the scooped-out zucchini flesh and set aside.
Heat the olive oil in a skillet over medium heat. Add the ground beef and cook until browned, breaking it up as it cooks. Drain any excess fat.
Add the chopped onion and cook until soft, about 3 minutes.
Stir in the taco seasoning and water. Bring to a simmer and cook until the liquid mostly evaporates, about 5 minutes.
Remove the skillet from the heat. Stir in the chopped zucchini flesh, 3/4 cup of the cheddar cheese, cream cheese, and sour cream until combined.
Spoon the taco mixture evenly into the hollowed-out zucchini shells.
Place the stuffed zucchini boats in the prepared baking dish. Sprinkle the remaining 1/4 cup of cheddar cheese over the tops.
Bake for 20 to 25 minutes, or until the zucchini is tender and the cheese is melted and bubbly.
Notes
For extra flavor, add 1/4 teaspoon of cumin or a dash of hot sauce to the meat mixture.
You can substitute ground turkey or ground chicken for the ground beef.
If you prefer a crispier zucchini, bake the empty shells for 5 minutes before stuffing them.