The requirements for any fitness work differ by what you want to do and where you are. Do you want to be a coach in an elementary or secondary school? Do you want to work with college teams? Do you want to work at a Y or a health club? Do you want a certification to teach water fitness, or to teach swim classes, or to coach a team?
I cannot see your profile, so don’t know your location, but if you are in the US one of the best places to start looking is with the Red Cross:
An easy way to begin working toward your goal is to contact coaches and instructors in your area and ask them if they would be interested in mentoring you. Follow that request up for the ones that are not interested with “Could you please direct me to a good starting point or someone you would recommend?” And be steadfast in your search. Eventually, you will get to someone who can help you.
The Red Cross does have Water Safety Instructor courses and swim instructor courses that often are required to move up the instructor/coach ladder in the aquatics world. But once you are a “known” commodity in your community, this become less of an issue. Many coaches do not renew their WSI cert once they are working, unless it is required for their employment (as it would be at most rec centers, etc.).
If you are interested in coming to Kona on the Big Island of Hawaii, I do teach coaching and swim instruction for all levels (age group, masters, high school, college, elite). This goes beyond WSI and most team coaching skills, knowledge, and abilities. You can get in touch with me through my profile.