They will not be allowed.
You can but try, but now that we are treated as on the same level as terrorists I think you may be dissapointed. As for your question on 'as it ever been known for visitors to be allowed on board' yes is the answer. In fact if you were in port next to a 'competitors cruise ship' you went to the officer at the bottom of the gangplank, said, 'I'm off the such & such ship, can I have a look around'. Yes Sir, they're just serving lunch, help yourself. It was their way of encouraging you to choose their line next time, but those days have gone now that we've allowed the cretins to take over the world. Good Luck in your quest though.
I should have pointed out I was reffering to the 60's & 70's.