Technically, the Bay is NOT a 'no discharge zone.' It is illegal to dump a tank or flush a
toilet directly
overboard in the Bay and all other US waters, but the discharge of TREATED waste from a USCG certified Type I or II MSD is
legal everywhere on the Bay except for one small harbor: Herring Bay.
However that information doesn't answer your question. Yes, sludge left to sit in the tank overboard discharge line will harden and block the line. And yes, it is illegal to flush it out. Since you can't use it until/unless you leave the Bay, you have a couple of options: 1. put a shutoff valve on that tank fitting.
2. disconnect the hose and
plug the tank fitting. Threaded plugs are readily available from most
hardware store
plumbing departments...wrapping it in plenty of teflon tape will prevent it from leaking. You can always re-install if/when you can use it again.