Preheat oven to 400°F. Grease a 9x9-inch baking dish.
In a skillet, cook ground beef over medium heat until browned. Drain excess fat.
Add taco seasoning and water, stir, and simmer for 5 minutes.
Stir in corn and Rotel tomatoes. Simmer for 2 more minutes, then remove from heat.
Prepare cornbread batter by mixing the cornbread mix, milk, and egg in a bowl.
Spread the beef mixture into the baking dish, then pour cornbread batter evenly over the top.
Bake for 20-25 minutes or until cornbread is golden brown and set.
Sprinkle shredded cheese on top and bake for 5 more minutes until melted.
Let cool slightly, then serve topped with sour cream, lettuce, and diced tomatoes.