In a large saucepan, cook beef over medium-high heat until no longer pink, 5-7 minutes, breaking into crumbles; drain. Remove from pan; set aside., In the same saucepan, add oil, squash, zucchini and red pepper; cook and stir until crisp-tender, 5-7 minutes. Add beef, tomatoes, water, salt and pepper; bring to a boil. Reduce to a simmer; cook, stirring occasionally, until vegetables are tender, 5-8 minutes. Stir in cilantro to serve. If desired, top with sour cream.