Well, you need up-to-date statistics. Here are a few arrows for your quiver:
In 2004, approximately 5.8 million people in the US participated in aquatic exercise. By 2007, this number had increased to over 7.2 million.
Aquatic exercise continues to appeal most to females in their middle or senior years, although that is mutating with the developing athletic programming focus. The average age of the quintessential aquatic exerciser is almost 47 years old.
When casual aquatic exercise participants are removed from the sample, the average age of the “frequent participant” (those who reported exercising in water over 100 days/year) ascends to 57+ years. Nearly 75% of aquatic exercisers are female.
Excerpted from Salzman, A. "Aquatic Aftercare and Wellness". In Becker, B, Cole A (eds). Comprehensive Aquatic Therapy. 3rd edition. Lippincott, Williams & Wilkins. Pending publication 2009. Statistics compiled in part from 2008 Superstudy of Sports Participation (Aquatic Exercise Slice).