I just called and they told me if the sailing is after Sept 1, 2017, you get the 2% on the whole cruise value that you paid. For those of you in Canada, paying in USD to the Canadian Costco, the person I spoke to said they will take the USD dollar value, convert it to Canadian and give you 2% on that amount.
Of course, unlike the cashback gift cards, this 2% goes on that check you get in Dec. for the year. Still, it will definitely pay for the Executive membership.
Also, don't forget, your rental car, if booked through Costco, will give you a 2% cashback.