The Blue Lagoon is a beautiful hidden treasure in Huntsville TX. For around $30/person you are assigned your own designated area.
Blue Lagoon caters mostly, and at times exclusively, to scuba divers, and has a Strict limit of the number of persons allowed in the park on any given day (get there early, or accept getting turned away with grace and understanding!).
The staff can be a bit abrupt in manner, but recent reports are that they have loosened up somewhat. The nature experience more than compensates. Your best strategy is to bring all of your food and drink, plenty of sunscreen. Be prepared for the rough road leading to the lagoon.
Bottom line is get there early, call the day before to assure there will be availability (though even this is no guarantee), and bring cash, in case the card reader is acting up! After all this it is big-time worth it.