Just for the sake of stating the obvious - every cruise line will say the same thing about a problem you had onboard if you did not take steps to have it resolved while you were onboard "We're sorry, but there ius nothig we can do now."
You say you complained about the drainage in your shower, but I don't see that mentioned in your letter to Carnival. It just says the same happened nightly.
As far as the neighbors making noise and the kids in the pool - i DO agree with you. If you notified the line of a problem it is a copout for them to say they rely on parents and other guests to behave according to the rules.
The cruise contracts have specific rules saying you can be kicked off of a cruise for misbehaving. And we have experiences where people were kicked off of cruise ships for "disrupting" a cruise.
In those cases, I agree that you have every right to complain. In fact, in the future I would get the names of people you complained to and write Carnival to tell them these people did nothing to resolve your problems onboard.
It is a bit of a contradiction on the part of Carnival to say in the first paragraph "had you told us onboard we would have fixed it" and then to dismiss the complaints you did make onboard as if it was not their responsibility.
So, bottom line, I don't blame you for being upset. Maybe it is time to find a new cruise line. Certainly other cruise lines try harder to curtail late night noise and kids running amok.