I'm gonna be a corpsman. What are my chanced of combat?

I go to Navy Boot Camp in 2 weeks. I'm gonna be a corpsman. They tell me that I will be given a choice of getting marine duty, but I don't believe that. I would rather work on a ship than go with the Marines, but eather way will be fine with me. Some people tell me I am defenatly going to get marine duty. Am I?