Bring a large pot of salted water to boil. Preheat oven to 400 degrees. Place 1/4 cup of the salsa in the bottom of a large baking dish.
Using a melon baller or a small spoon, hollow out the center of the zucchini halves. Leave about 1/4-inch thick shell on each half. Dice the scooped out flesh of the zucchini in small pieces and set aside 3/4 of a cup to add to the taco filling. Use a paper towel to squeeze out the excess water. Discard the remaining scooped out zucchini or save to use in another recipe. Drop zucchini halves in boiling water and cook 1 to 2 minutes. Remove from water and set aside.
Brown and stir ground beef in a large skillet until crumbled. When no longer pink, add the taco seasoning and mix well. Add the onion, reserved zucchini, bell pepper, tomato sauce and water. Stir and cover, simmering on low for about 20 minutes.
Use a spoon to fill the hollowed zucchini boats with the taco meat. Divide the meat equally between the zucchini boats, about 1/3 cup in each. Press meat down firmly into the boats. Top each with about 1 tablespoon of shredded cheese. Cover with foil and bake 35 minutes until cheese is melted and zucchini is cooked through. Serve with salsa on the side.