Water aerobics is a great form of exercise and will surely be just as effective, if not more effective than walking. Walking is a great form of exercise and almost anyone can do it! The same goes for water aerobics! The impact on your joints is next to nothing as your resistance is the water. Tell me the last time you played in the water and were injured? Hopefully you will not come back with a time in which this occurred.
Swimming allows your body to work all muscle groups and to do so with the only resistance being you against the water. Walking in the water will be much more challenging than walking on land. When we walk on land our bodies are able to move with our own force but when you walk in the water your body is going against the water and the water will not make it easy. Also, water aerobics is a great way to get some resistance training and have some fun along the way!