Civil courts will entertain a suit against anyone, if there is enough of a theoretical case to survive a "motion for summary judgement" (rough translation--a reasonable person/juror could possibly find negligence, rule violation, and duty to act, etc).
Having a license, as mentioned above, could make your attempt to "not know any better" or "nothing to do with me, I was just the rail-sitter") sound less than genuine. I see this same issue in scuba, those with Instructor, or Rescue qual cards will keep quiet about it, and instructors who happen to be pleasure-diving may be asked why they didn't "do something" for some other diver who was not their buddy. I must say, this is almost as much a social question as it is a legal one, "when am I my brother's keeper?", or indeed, "who is my brother, anyway"?? But recreational boats (unlike, say, pleasure scuba) do have a "command structure", either stated or unstated, typically with the owner, if aboard, being the unlicensed "captain". So a "mere passenger" should not have to wonder if he/she will be made a "captain" by a judge or jury after the fact (note I said "should", some things can't be predicted).
On the Coast Guard side of things, I believe that any negligence or misconduct must be while "acting under the authority of your license" in order for them to have jurisdiction to proceed against that license for suspension or revocation. And years ago, I was one of those CG guys who could bring suspension proceedings against some license holder. Now I'm a license holder. So I've seen both sides, in a way.
Actually, I think Capecodda and Boasun just said the same thing, only better and shorter ;-)