Heat a large skillet over medium-high heat. Cook and stir beef in the hot skillet until browned and crumbly, 5 to 7 minutes; drain and discard grease.
Mix ground beef, tomatoes, rice, ketchup, Worcestershire sauce, and black pepper together in a bowl. Stuff each green bell pepper with ground beef mixture.
Arrange stuffed bell peppers in the crock of a slow cooker; place the reserved tops back onto each bell pepper. Pour water around the base of each stuffed bell pepper.
