The rules requiring passports were supposed to start yesterday, but last year the government decided to hold off until this coming spring - with no confirmed date for implementation when I last checked. You may recall the mess this past summer regarding the passport requirements for air travel...the cruise ship passport thing is shaping up to be the same mess.
Bottom line is that at some point in the future, a passport will be required for all cruises no matter where they go. You have time, so I strongly urge you to go ahead and get the passports now. That way you won't have to worry if the government suddenly decides to start enforcing the new rule this coming April. And they are good for 10 years, so it isn't as though it is a big expense.