Exercising in the water can be much better than out of the water. Less impact on the joints and more resistance to make you work harder. Swimming works just about every muscle in the body as well. My gym has what they call a lazy river where the water is moving somewhat rapidly. I walk it in reverse where it becomes resistive and have found it burns more than any other exercise I do.
Water aerobics is like any other kind of cardio, like running or biking. It can definitely help you lose weight, since you burn calories doing it, but you'll need to make it intense (burn enough calories) and watch you diet as well for the best results.
The easiest way to determine if water aerobics burn enough calories is to do them for a week or two and see if you lose weight. If that's not enough, start counting calories and making sure that you're burning more calories than you're taking in.
Losing weight is a science, but also an art in adapting it to yourself and your schedule/dedication.
