do you have to be in the military before you can go rescue swimmer. do you have to be usa citian to join

Yes and no.

You have to join the Coast Guard to go to AST "A" school. The Aviation Recue Swimmer program is part of Aviation Survival Technician (AST) "A" school.

You can join the Coast Guard without being a citizen. However there are conditions that govern this. One of them is that you can not go into an aviation rating. A recruiter would be able to tell you what ratings (career fields) are available to you.