Water Park sizes tend to vary in size from being small with a couple of slides to very large and stretching out across multiple acres.
Some of the slides and pool formations can be used as barriers to oncoming zombies. The higher structures are good for seeing the surrounding area and outward for long distances.
There are concession stands scattered around the park thus having access to not only food, but also the stoves and fridges used for cooking and storing the food.
The locker rooms can be used as a small storage area.
Just like an Amusement Park, it can be time and resource consuming to secure a Water Park.
There may be a lot of blood contaminating the pool water making it extremely unsafe to swim because of the possibility of it being tainted.
Even if no bodies have made contact with the water, its high levels of chlorine make it undrinkable.
The large open spaces make it easy to be seen by zombies and bandits.
Most tend to be within towns, and cities. Bringing major amounts of danger to any who decide to make the park into a base.