I would strongly recommend that you do not teach water exercise without any training. When people attend a water exercise class, they entrust their health and well-being to the group leader, and this is a huge responsibility to take on. Lack of knowledge puts everyone at risk, including the instructor. My recommendation to people who want to make the transition to being an instructor, is to attach yourself to an experienced, competent instructor as an intern. Work with another instructor who will gradually give you teaching assignments, test your knowledge as you learn, and provide supervised team teaching opportunities.
Training is an ongoing process.
There are so many variables to be addressed just in water exercise. You may only need one Certification, if you pick the right one. You may not need a Certification at all. It will vary for each individual. An instructor with no exercise science background should do a certification.