You are correct. An 18 month old will not be allowed in Camp Carnival. They are very strict about the 2 year old minimum.
If the child is completely potty trained by then (no..swimmies are not allowed) they would allowed in the pool.
Camp Carnival does offer babysitting at Camp Carnival from 10PM - 3AM for $6 an hour for ANY age.
Personally, I would never do it unless I knew for certain that my child could handle the travel.
Every child is different. You know if the child handles travel well or not. If they do, then it will just be a matter of caring for the child 24/7. If they don't, then it will be a matter of dealing with a crying child whose schedule and surroundings are ascew....and dealing with neighbors who are not going to appreciate having their cruise ruined by irresponsible parents.
I would suggest that this child's parents get a room with a balcony. That way, the parents can relax on the balcony during the child's nap time.
I would hope the parents will be respectful of their neighbors because the walls are way to thin.
As the mother of a special needs child, our system has always worked well because I was smart enough to realize his limitations and work within those boundaries.
Dealing with a toddler requires the same attention.