The designated smoking areas are listed in your Capers every day.
When Carnival made the change to designated smoking areas, it was mainly to stop folks from walking from venue to venue with a lit cigarette.
So as long as you stay in the designated smoking areas, you will be fine.
Yes, you can smoke in your cabin. However, be aware that smoke in the hallways is not allowed.
So you can't open up your cabin door to "air out" your cabin into the hallways.
Generally, you can count on the Piano Bar, Casino and Disco on smaller ships to be the designated smoking areas. On larger ships, add the Cigar Bar and Sports Bar to the list.
Also one side of the pool area outside. (usually starboard)
Have a great cruise!