Smoke outdoors somewhere near the aft end as long as it is a no-smoking area. The fewer people nearby-the better, but don't be obvious about staying distant. Try walking around the outside decks, and you'll find certain times are less crowded than others.
Be cool and act like it's nothing but a cigarette that you are smoking and nobody should give you a second glance, unless you start coughing or somking it while pinched between thumb and forefinger.

I do it everywhere I go:
cruise ships
fishing charters
walking downtown sidewalks
waiting outside while the wife gets her hair done, goes shopping, etc.

I refuse to be thwarted.