Latte31 - I think that performing exercises in the water are also great for those new to exercise or that have a significant amount of weight to lose.
It is fun and it is pretty difficult to sprain a muscle performing water aerobics or swimming and it does burn a lot of calories as well as conditioning your muscles.
Even if you simply march in place you can make a difference in your leg muscles. Being in the water also makes you cooler which is a little more comfortable than working up a sweat outside of the pool.
You can also use your arms and in an alternating criss cross fashion to condition your arms while you lift your legs to work your leg muscles. The pool is really versatile and an easy place to condition your muscles.