Water can be used as a substitute for sake in teriyaki sauce, but keep in mind that sake contributes not only to the flavor but also to the overall complexity of the sauce. Sake brings a unique depth and a touch of sweetness that water alone might not replicate perfectly. If you're using water as a substitute, you might want to adjust the other ingredients slightly to make up for the missing flavors. Consider adding a touch more sweetness, such as mirin, to help approximate the taste of sake.