many organizations will certify you for aquatic fitness but my vote would go to the Aquatic Exercise Association http://www.aeawave.com/PublicPages/Certification/AEACertification.aspx which has a very good reputation. My reason is that they are specializing in water fitness rather it being one of many options.
Wish you good luck.
Hello Cheryl Williams,
You can also check out the AEA as suggested by Karin at: