NCIS is actually a civilian Law Enforcement Branch with in the Dept. of the Navy. All NCIS Agents are GS and not actual military personnel.
It's my understanding that the Marine Corps has all but done away with the Criminal Investigation billets (CID) turned most if not all investigative duties over to NCIS, both in CONUS and aboard.
CID in the Corps was always manned by mostly SNCO's and WO's anyway so a posting to CID was not a one hitch deal.
NCIS is also going to want a degree so college is pretty much a must. I'd stay away from Criminal Justice however and maybe concentrate ob Business, Computers etc.