I'm kind of split on this one. On one hand, I don't think it's wrong for Mom to save a chair for Dad while he parks the car, runs to the restroom, arranges lockers, etc. On the other hand, I think it's extremely inconsiderate to save a chair for everyone in your party when it's unlikely that they'll all want to sit down together for an extended period of time.
I don't think there is really an enforceable policy here. If you have a blanket "the seat is only 'taken' if someone is in it" policy, then customers won't be able to make trips to the bathroom, the concession stand, or to take a ride or two with the kids. That would probably deter entire families (why would Mom and Dad pay for tickets for the entire family if they didn't have a comfortable place to relax while the kids are off playing?).
The problem with saying "no chair-saving" is that couples and families generally want to sit together, and people don't usually want to sit next to strangers. If the only seats available are staggered, you'll run into essentially the same problem you would with the "towel-reserved" seats. . . no places suitable for you as a couple.
I think the best solution to the problem would be having cabanas or seating areas for rent. That way, a family that wants to have six chairs available could pay $X to rent six seats for the entire day. Nobody else could try to borrow or "reserve" them, and Mom could park herself there to watch the stuff.
It's definitely rude to "reserve" several chairs on the off-chance that you might have to use them all at the same time, but I don't object to someone wanting to sit with the rest of their family when taking a break from swimming to eat, sun, or rest.