Dont know a ton about Corpsman, but i do have to agree with comment posted above. Run, Run, Run.. Marines are hardcore PT types.. and boot camp isnt that tough. I went through 8 years ago, but just got back from great lakes in May where i was in school for 4 months, and I talked to some of the new kids over there. Boot Camp is almost ridiculously easy now.. Your galley (chow hall), classroom, and barracks are all in the same building now, so theres minimal marching, and all PT is done in sneakers now, no boots. And the RDC's (drill insstructors), cant be as mean now as they used to be. (political correctness). But I do not regret being in.. The navy has been good to me..
As far as being a combat corpsman, or as the navy calls it, Fleet Marine Force, its a single tour, as far as i know, meaning after you have done an enlistment, or a few years there, you will be rotating out and back to the regular fleet, or one of the navy hospitals. Actually, if you wnna know details, call one of the navy hospitals, and ask to speak to one of the FMF corpsman there. Just tell them what you wanna do, and im sure they would be happy to tell you aout it. Portsmouth Naval Hospital, or Bethesda, etc..