I have a cruise reservation with Celebrity thru a TA (who's with Cruiseone, not that it matters) and had her try a $1 payment using my Cash Rewards card that has online as the 3% category. The TA entered the payment directly on Celebrity's TA system. The payment showed up as Celebrity Cruises and coded as an online purchase and I got my 5.25% (I am the top tier in their rewards program). That means I'll be able to use both my CR card set for travel as well as the one set for online purchases to make payments. I need to max out my $2500 in charges for this quarter on both cards since I owe a bunch for B2B cruises I'm doing next year and paying everything in 4q will max out the category.

Since all cruise payments I've ever made have shown up as a charge from the cruise line itself, hopefully this is the way that other lines will also work, but I guess someone needs to try it. If you booked yourself, you should be able to make a minimal payment yourself on the cruise line's website to see if it codes as online.