I was a commercial fisherman for almost a decade. It required a very substantial amount of physical effort.
As long as you did swimming as a sport in highschool and did reasonably well you should be fine.
It's possible it might require a degree in physical education but that all depends on the program.
As for the interview, just be yourself, be honest, and no matter what happens you won't have anything to regret. It sounds like you have a personal relationship with the coach which will help more than most of your qualifications will. Sounds like you should do fine.
Good luck
