There's basically no reason not to try this - HOWEVER -
Be prepared to politely swear that you didn't know it was against policy and happily surrender it if you are caught!
As far as I know, if you do this they won't deny boarding to you. (If the contraband was actually something illegal i.e. drugs or weapons, then you'd probably be in trouble with the Port Authority and you'd likely miss your boat.)
I don't know of anyone *firsthand* that's been caught, but I've seen the pile of "contraband" on boarding days! So people do get busted for this. Just don't make the slightest fuss about it if you do, cooperate completely, and you'll most likely be fine.