Marine Vet trying to join the FBI or NCIS without a degree. Any advice?

Get the degree. You didn't take any shortcuts as a Marine, don't start now.

NCIS has non-agent billets that you might be eligible for. But if you want to be an agent, unless you already have law enforcement experience, you certainly won't be competitive. So if you do want to be an agent, get a degree and/or get a job in law enforcement after. Here's a list of Frequently Asked Questions about NCIS Special Agent applications:

http://www.ncis.navy.mil/Pages/SAFAQs.aspx

Get a degree. FBI won't take you as an agent without one.