I am curious about this as well, I will ask my friend as she recently booked a cruise on her Sapphire.
Gardening until September 2025Great question. I'm interested in the eventual answer to this as well, as I'm cruising in September. 132 day! (but who's counting) 
When I've booked cruises (16 so far), it's always counted as "Travel." This has been true for the CapOne Venture (was able to redeem as a travel charge), CSP (earned 2URP/$ as travel), Costco AmEx (2% for travel), and Discover (5% when Travel was the quarterly category). Note that every time I've done this, whether paying the cruise line directly or paying through a travel agent, the charge always came through as "CELEBRITY CRUISES" (or whatever the cruise line was) and coded accordingly.
Don't forget that when you check in to the boat, you'll provide a credit card for settlement of your account (tips, tours, booze, etc.), and any resulting charge will also code as travel, so use the right card there as well. Note that at one time, on at least one cruise line, you could get cash from the casino (presumably for use in gambling, but not required) using your ship card, and it would go on your account, thus providing a convenient way to earn lots of points, miles or cashback for no actual spending. Regrettably, that MS exploit is no longer available from any line, AFAIK.
Also, not that you asked but FWIW, in my experience, airfare booked through a cruise line (e.g. Celebrity's Choice Air) books not as airfare, but as cruise. (Sadly, I chose the PenFed AmEx for payment when we recently booked fares through Choice Air, hoping it would pass through as Airfare for 5x points, but only got 1x. 
)
I'm thinking that the Citi Costco Visa will be my new go-to card for cruise payments, at 3% CB.
Chris.
@Anonymous wrote:When I've booked cruises (16 so far), it's always counted as "Travel." This has been true for the CapOne Venture (was able to redeem as a travel charge), CSP (earned 2URP/$ as travel), Costco AmEx (2% for travel), and Discover (5% when Travel was the quarterly category). Note that every time I've done this, whether paying the cruise line directly or paying through a travel agent, the charge always came through as "CELEBRITY CRUISES" (or whatever the cruise line was) and coded accordingly.
Don't forget that when you check in to the boat, you'll provide a credit card for settlement of your account (tips, tours, booze, etc.), and any resulting charge will also code as travel, so use the right card there as well. Note that at one time, on at least one cruise line, you could get cash from the casino (presumably for use in gambling, but not required) using your ship card, and it would go on your account, thus providing a convenient way to earn lots of points, miles or cashback for no actual spending. Regrettably, that MS exploit is no longer available from any line, AFAIK.
Also, not that you asked but FWIW, in my experience, airfare booked through a cruise line (e.g. Celebrity's Choice Air) books not as airfare, but as cruise. (Sadly, I chose the PenFed AmEx for payment when we recently booked fares through Choice Air, hoping it would pass through as Airfare for 5x points, but only got 1x.
I'm thinking that the Citi Costco Visa will be my new go-to card for cruise payments, at 3% CB.
Chris.
Really good stuff! Sounds like you're the one to chat-up when I'm ready to pick a cruise. (It was supposed to be Alaska this summer for the big birthday I'm having, but since DH wasn't up to big travel at this time, we got a puppy instead.)