miesposo told me that when he was cycling around costambar the other day he saw a nekkid "ruskie" girl ("very pretty, good body") by the side of the pool in one of the apartment buildings. she was a bit drunk and tried to invite him to join her.
so maybe, if you insist, any pool can be private
cabanas may not really be that much cheaper than the price by anhvee (unless it's per person per night). if i remember correctly la roca charges 800-1000 extra for the use of the pool on the top of their 4 hour rate. maybe if you rented the place for a day or two you can negotiate final charges, i don't know.
the alternative is a rental on any small villa/house on the north coast that has a pool. you cannot get more private than that. airbnb has a wide range of prices.