I had a "minor" problem with my last Carnival cruise and sent a letter to them thru planetfeedback.com....got a letter in the mail apologizing and a shipboard credit of 25.00 plus a free gift basket on my next cruise.
Be prepared to only get a shipboard credit or a discount on your next cruise. Therefore if you don't want to ever sail Carnival again it may not be worth it as you'll have to use the credits on your next CARNIVAL cruise.
I've been very please with planetfeedback.com on other companies too. I wish you the best of luck. And be positive!