Generally speaking, water doesn't make a great substitute for milk in a breakfast casserole (or in many other things, for that matter). The reason is that milk doesn't just add moisture, it also adds fat. u003cbr/u003eu003cbr/u003eFat helps to make baked goods more tender and this breakfast casserole richer. If you don't want to use regular milk, feel free to swap in non-dairy milk. You can also liven up your water by adding about a tablespoon of vegan butter or oil per cup of water you use.