There's a "Check Availability" button on the WDW web page for the cruise that takes you to a calendar ... looking pretty much identical to the booking engine for ADRs. If you click that button, you can then click to select a date ... and it then asks you to "Select the Number of Guests". Then, when you click on "Next", it displays available times. (Not surprisingly, there doesn't appear to be any availability over the next 60 days.)
Are they not bookable via that webpage? (i.e. Going through that 'select a date, select the number of guests' process doesn't work?)