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: