There are new offers from Celebrity Cruises. If you book a trip with Celebrity Cruises you will get a very generous return on your spend. If you have already paid for a cruise, there is a chance that you can buy onboard credit ahead of time and still get a bonus Membership Reward.
If you were planning on booking a cruise with Celebrity Cruises anyway, these Amex offers are an excellent deal if you only value membership rewards at 1cpp. If your partner has the offer on any of their cards, you can pay with two cards with offers loaded to them to double up on the membership rewards.
It is possible that an alternative option will work as well. The Celebrity Cruises website allows you to pre-pay for onboard credit online. You need to do several transactions in order to reach the threshold of $500 or $1,000, because the only denominations available are $25 and 9. The multiple transactions aspect won't be a problem, but there's no guarantee that they'll code correctly in order to get the bonus Membership rewards.
The best way to take advantage of the Amex offers is to pay for your cruise. If you paid for your cruise with an American Express card, Celebrity Cruises may be able to give you a partial refund if you use one or more of their Amex cards. I would be inclined to buy those onboard credits if I were you. If you don't earn the bonus membership rewards from these Amex offers, there's not much of a loss other than the bonus points you could've earned by putting the spend on a different card