Whenever I read an independent article in a travel publication or an online source, it usually advises to buy the
travel insurance from a source other than the travel vendor, to avoid any potential conflicts with the provider.
I always buy the insurance when I book the cruise for the benefit of using the provision that will cover existing medical conditions. Often, if you don't buy it within the first 7 days of booking the cruise, the insurance will not cover preexisting conditions.
Technically, you can wait until the day before your cruise to purchase the
trip insurance, if you buy it from some companies, like CSA, available online.
http://www.csatravelprotection.com/insure/default.ssc
phone 800-873-9855
I can't offer any exerience using any insurance other than the one that
AAA uses when you book thru them, I just know that the CSA people will book very close before your cruise.
I think I read from other posters that you can purchase the insurance thru
DCL up until the final payment date.
hope that helps