For what it is worth... the cabanas will EVENTUALLY be a great add-on to Volcano Bay, however they are not ready for prime time based on my experience this past weekend.
I would not be surprised if they chose to eliminate pre-booking cabanas at this time until they get the situation under control. I'm sure they will have no problem selling whatever inventory they have on the day of... and it allows them time to reassess daily how many units are fully operational. Good operating strategy!
My cabana experience included spending 1.5 hours being shuffled around the park in search of the what was our assigned cabana only to be settled into one that did not match the description sold at the time of booking. The tapu tapu kiosks are not operating as of yet, the refrigerator (while present) had no electrical connection nor did the in-cabana ceiling fan. Additionally there is no way to contact an attendant, food delivery was haphazard and it required a one hour wait just to close out the tab at the end of the day. This is a premium product that has great potential but like other elements of the park are just not 100% ready.
This is a beautiful park that is definitely experiencing major teething issues. I was part of the opening team of USF, and while this is reminiscent, it is not as bad. It is quality with operating attractions that are truly great (aquacoaster!) ! Just needs some time to work out the kinks and increase attraction capacities. It will happen with time. It took us a good 3-4 months to get my home attraction at USF close to it's designed theoretical operating capacity.
I wish them luck... team members were working their b****'s off to make everyone happy. Very commendable considering the rough start.