I'll be up front here: I'm Air Force not Navy. My knowledge is general military healthcare.
Nurses commission as an officer (not enlist). Also, the vast majority of nurses in all branches practice in a brick and mortal hospital/clinic environment. There are opportunities to do things in more austere, dangerous environments (ex. forward surgical teams). These are generally experienced ICU, ER, and OR nurses. Most nurses, however, treat patients in a stateside hospital. When they are deployed, they are typically sent to a field hospital within an established overseas base. That isn't to say that nurses are 'safe' when deployed.
I want to caution you that in general nurses aren't field medics running around outside the wire with a squad of spec ops guys. If you want that, you should really look at PA or some of the enlisted career fields (ex. AF Pararescue).