To be the safest, I'd take the rubber hose off and plug that elbo. Then I'd cut a plug to knock into the mushroom thru-hull from the exterior making sure that it does not reach the gate valve inside. Winterize your system and address the thru-hull and new valve at spring haul-out.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.