Combine the teriyaki sauce, water, cornstarch, and sugar in a small bowl. Stir to combine.
Spray a large skillet with nonstick cooking spray. Cook the beef strips in the skillet over medium-high heat for 7 to 8 minutes, stirring occasionally.
Add the sauce, frozen vegetables, and water chestnuts to the skillet and bring to a boil. Quickly reduce the heat to medium and cook for 7 to 10 minutes or until the vegetables are heated through, stirring occasionally. Remove from the heat and serve on a bed of rice.