I know they would have to provide extra towels (you get 4 for the base price, I am assuming they would add additional towels for the supplementary guests).
An all day cabana rental for six guests (which includes rental towels for everyone, a large locker inside your cabana, your own fridge, 12 bottles of Dasani water, and 20% off all retail purchases) for $80 is a STEAL in comparison to cabana rentals at either Disney waterpark ($345.06 including tax, regardless of the season!!) or even Wet 'n Wild.
We have rented cabanas at Aquatica on each of our visits, and they are worth the price you are seeing and more. I think your price for six is excellent.