Hi, Gyggles / Laura! I've been an aqua aerobics instructor for over 8-1/2 years now, and it's still fresh, fun and totally effective. If I do say so myself, I'm a helluvan instructor, and over the years I've been able to get the most out of the pool environment that has nothing to do with traditional swimming.
I teach two classes per week, Wed. p.m. and Sat. a.m., and on Saturdays after class I put in some solo pool time just for me. I'd estimate water training, including the classes I teach (and I'm in the pool doing the class as well as leading it) and my solo work, comprises about 40% of my overall exercise program. So it's an integral part of my routine, not an extra.
IMHO, aquatic exercise in its many forms is a perfect cross-training mode, and actually is tailor-made for Cathe exercisers. There are many here who are concerned about impact forces, and the reduced-impact quality of traditional shallow-water aqua aerobics is a great alternative. And there is a great deal you can do, including traditional swimming, prone and supine kicking drills, and aqua jogging in a zero-impact environment as well. And of course the continual resistance AROUND the body makes it a looking-glass-world training environment. There's no way I could do the mish-moshes that I do on a regular basis if I didn't have the water stuff to complement things.
WHEW! Long answer to a short question. It can be "enough" IF you understand the water environment and how to manipulate your motions for maximum workload and resistance. It can be enough as a PART of your routine, but unless there is a specific orthopedic reason otherwise, I do not recommend anyone rely on water fitness alone for their exercise. You need the bone-density-promoting qualities of landed, weight-bearing work, and aqua is good for muscle endurance, not strength.
A-JOck