I was in another agency, but had dealings with both Air Marshals and U. S. Marshals.
As I am sure you are aware, no one loves
their job all the time. Having said that, most of the Air Marshals and U. S. Marshals I've met and interfaced with seemed to be content with their job.
Note those are two very different job descriptions.
Check out www.usajobs.gov
USAJOBS - The Federal Governments Official Jobs Site. The site has job openings, job descriptions, pre-requisites and a computerized application system. Pretty much all you want to know.
Good fortune to you in any field of endeavor, and thanks for serving.