In general, I found that the staff comes from three main sources. There are human performance investigators that come from academia, structural and other engineers that come from professional and academic backgrounds, and operational experts that come from the industry in question, be it aviation, rail, road, or marine.
Looking around the web, I wasn't able to find a ton of information about the marine staff at NTSB. I was able to find an account from 2003, written by the current Senior Human Performance Investigator at NTSB of how he got the job there. I have linked to it below.
Trying to step out of the trap of constant googling, I tried calling the Seattle branch office of the NTSB. Though they appear to be an aviation branch, I hoped that I could finagle a short informational interview. As of now, no such luck.
Through my research, I began to realize that getting a job at the NTSB would be akin to trying to get a job as an astronaut. It might be a good final goal, but the real meat and potatoes of the career would be the long(ish?) journey there... This in mind, I began to broaden my search, looking for the smaller players in the field, the intermediate steps, the places people use and develop the same investigatory skills--just in less high profile cases.
My next few posts will cover my research into those smaller players: USCG investigators, insurance claims adjusters, marine surveyors, maritime lawyers, etc.