I know you used to be able to use them on Renessaince cruises but I think they went out of business last year. I don't see any cruise lines listed as partners but some of the hotel programs let you use their points for cruises.

You can transfer your miles for Hilton HHonors points and then use those points for Carnival and Princess cruises. You can also transfer miles for Diners Club miles and use them on several different cruise lines.

Not sure how worthwhile it is to do this though.