I believe the only "big" cruise lines included in the $300 OBC is Norwegian, Holland, and Celebrity. Even then you have to book a suite to get the $300OBC, for a lower stateroom you will get a same class upgrade (i.e. low end balcony to high end balcony). The other amenities included are a free dinner for 2 at one of the pay for restaurants, and a bottle of champange/wine.

Also just got back from a Holland cruise and my Amex TA told me that there is a one time annual benefit of $100 Amex statement credit for booking the cruise, never bothered to ask what type of cruise qualifies.

Lastly, not sure if this is a plat benefit or just a benefit for booking with Amex, if the room you book is in excess of $1,200 a person or $1,950 for single ocupancy you get a free seven night stay at Pueblo bonito resort in cabo/matazlan. Used the cert last year and it was great. Hope this helps

Go gators