www.americanexpress.co.uk/claims
Allows you to input your card details to generate an insurance certificate and check levels of cover. I'm pretty certain unless you've covered the costs of your cruise with your Platinum card you won't have the full coverage in terms of cancellation. You should still be ok for medical/emergency but you're best advised to call the contact on the link provided and have everything clarified - this is to the underwriter AXA who facilitate the Amex Platinum insurance policy.