Actually you have 2 free cruises coming your way. The first one of course is the cruise certificate you won on the cruise for reaching the points needed on a single cruise. Your certificate will list the cruises on the back that you can choose from and the time frame you have to take it in.
The second cruise is for reaching Prime which entitles you to a free cruise to be taken between April 1st 2019 and March 31st 2020. You will receive a certificate after April 1st for that cruise. As already stated it can be 8 nights or less and not over a holiday.
You could possibly use the second certificate for the Alaska cruise. Call the number on your certificate and enquire, but be clear you are not calling on the existing free cruise but for the second you'll receive in April.