Preheat your oven to 400 degrees Fahrenheit (200 degrees Celsius). 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 large skillet over medium heat. Add the chopped onion and cook until soft, about 3 minutes.
Add the ground meat to the skillet. Cook, breaking it up with a spoon, until browned. Drain off any excess fat.
Stir in the taco seasoning, water, and the reserved chopped zucchini flesh. Bring the mixture to a simmer and cook for 5 minutes, allowing the liquid to reduce slightly.
Remove the skillet from the heat. Stir in the sour cream and half of the shredded cheese until combined.
Spoon the meat mixture evenly into the hollowed zucchini shells.
Top each boat with the remaining shredded cheese.
Place the stuffed zucchini boats in the prepared baking dish. Bake for 20 to 25 minutes, or until the zucchini is tender and the cheese is melted and lightly browned.
Garnish with fresh cilantro before serving.
Notes
If you prefer a creamier filling, mix 2 ounces of cream cheese into the meat mixture before stuffing the zucchini.
For extra flavor, add 1/4 cup of diced green chilies to the meat filling.
You can substitute ground turkey for ground beef to lower the fat content slightly.