Get legal advice from your lawyer first, then fight them.

If your lawyer is positive that your travel documents are complete, can ask him/her send an attorney's letter to the cruise company. Most case they will cave in and pay back your cost. If they budge, you can then file a civil suit in the civil court -- make sure your lawyer can convince you that there is no grey area that cruise company may also score.

Worst case take it easy -- $3500 is not a small amount, but not big enough to ruin whole family's mood/life as well.