Deck gun is buggy, yes. For the moment, man it yourself. 2 crewmembers should pop up to man the gun. When you leave it, they won't fire since you never gave the order to man it.
F5 toggles man/secure gun. Telling the watch officer to man it and using F5 to secure should work, might be a bit flaky though. As far as I know, morale doesn't come into it; always been able to use it at will.
