Where you see sings for smokin.
Keep your eye on the web site of the cruise line you are interested in because the rules are changing almost weekly. Royal Caribbean, for instance has made some quite stringent changes recently. I think they have been waiting to see how many restrictions they can place without getting too many recriminations. Rumour has it that one line is even thinking of trying out a small boat towed behind the main ship, with a built-in shelter of course, especially for smokers .
Its not registered in the uk so uk legislation does not apply however they do restrict smoking in certain area.