Kind of a long story here, but bear with me

I took a Carnival cruise just a week after the Carnival Triumph debacle in the Gulf Of Mexico.  I was booked on the Carnival Destiny (now the Carnival Sunshine).

A few days before departure, I received notice that the ship was experiencing some mechanical problems, cutting short it's itinerary and returning to port for repairs.  They did not expect any issues but two of the ports of call might be cancelled.

Day of departure, we were told that the port thrusters were not going to be repaired in time, and they would be cancelling two ports because they did not have tugs at the dock that would be required to assist.  Plus we departed 6 hours late.

Cancelled were Turks & Caicos and Half Moon Cay. Inserted in it's place was Key West and an extra "Fun Day At Sea".  Our compensation? A $50 on board credit. 

I was astounded at the poor "compensation".  This was not what I signed up for.  I'm sure there was some small print somewhere in the contract that said this was perfectly OK.  I think $50.00 that I had to spend on the ship was a joke.  

Has anybody ever had ports cancelled, and what was your compensation?  What is the industry standard?

This is the only issue I've ever had with Carnival and have cruised with them since.  I just would like to hear of others experiences when dealing with port cancellations.