ok there are quite a few things that i want to make clear. the marines are under the department of the navy and in such are considered a SF within the Marine Corps there are to elite groups, Marine recon and Marine Force Recon, those are about relative to Delta force. USAF has a SF that i am aware of called the PJ's (pararescue jumpers) they are airborne medics. If u look at seal of the Marine Corps u will see Dept of the Navy USMC the marines are not their own branch. also the correct term is not drill intstuctor not seargent that is not used anymore. Also the people that train the Officer canidates at OCS (officer canidate school) and also at USNA is a mix of officers and enlisted NCOs. I have the greatest respect for Navy Corpman as they will be watching over me in a few years when i go to Parris Island but they are not doctors they are there to perform basic battlefield triage then get the wounded to the field hospital. and they do an amazing job.
Semper Fidelis