When you book a cabana, you can have up to 6 people (or pay extra to have more included). They wouldn't limit who could book it (i.e. a single cruiser), they have no idea if you're going to be including people from different cabins in your cabana.
Even if there's not one available when you do the online check in, continue to watch the excursion page - you never know when someone will cancel.
I think it would be fantastic to have a cabana all to myself for the day.