You sure can take a 18 months old toddler on board,you do however will not be able to take advantage of long list of activities provided onboard. I know most cruise line have kids center on the ship, however most if not all of them have age limit of 3yrs old.I believer the only exception is Disney cruise. So you will have to care for your child all the time, and that means someone has to sacrifice his/her enjoyment. Another issue you may have is with dinner. You are more likely to eat at the buffet place instead of formal dinning room due to time constraint, kid behavior, etc.