I don't know if this helps.. but I'll offer it up.
In my personal experience, the gift cards (once redeemed) are exactly just that... Gift cards. They can be used for anything you can use a gift card for (in this case, an OBB). The connection to AMEX is severed once the gift card is purchased/redeemed.
However, personally, I think that AMEX points being used for gift card purchases is not the optimal way of redeeming valuable AMEX points. You may want to consider seeing this guy's blog on effective ways to use points.. It's one of my favorites:
http://thepointsguy.com. He does a monthly valuation of points (currently rating AMEX about $.02 per point compared to the $.01 per point a gift card purchase gives you). He also goes over justification for his valuation and suggestions for redemptions.
Personally, I miss living in Ohio. My strategy used to be that when I was in Ohio, I'd wait for a quarterly promotion at Giant Eagle for free gas on Gift cards (with no limit). Then, I'd use my 5 points per dollar at grocery store AMEX credit card to stock up on enough Disney gift cards to get a free tank of gas (buying Disney cards and a single candy bar to ensure the transaction was coded as "grocery"). Then, I'd be getting 10% in terms of AMEX points valuation in addition to a free tank of gas every quarter.
Now that I'm in California, the best I can get is Target, and I'm not a fan of that process.
I hope this helps.
However you choose to use your points, enjoy your cruise!