Okay, so I just got back from driving SR 519's route this afternoon, and I can confirm I saw that one sign, but not any others. I'm still unsure about including it, as I don't see any other signs, not even a BEGIN 519 sign on Edgar Martinez Drive or anything like that. Though, if you follow the "Ferries" signs, they pretty much take you on this state route.
I still think I'm going to leave it out for now. There's not enough signs to enable a driver to easily follow this route.