You were lied to?
Again - not to irritate you, but:
a) What was the lie?
b) How do you KNOW (not think) it was a lie?
c) What SHOULD they have done differently?
d) Do you have an 'inside contact' in Carnival's ship maintenance that gave you the 'scoop' on the parts stockage levels in Cozumel and/or the maintenance history and configurations of those drive systems. Many assumptions are being made.
Perhaps Carnival SHOULD have done more for the guests inconvenienced by this unforeseen occurence, but it is now up to YOU to formally write to the senior management of Carnival and intelligently convey your case. The pursers and other personnel onboard the ship were trying to handle a potentially volitile situation in dealing with over 2,000 upset passengers and probably issues your small credit (if any) as a preemptive measure. I'm 99% confident that if you wrote an email to the approriate executive and were constructive in your message - you would be reimbursed in some way for your inconvenience.
Again, my $.02