I hate to throw water on any of the parks, but they can't ALL be the largest. Let's sort it out and make some sense about the claims. Are you ready to dive in and get to the bottom of this? By "largest," I am going to focus on the square feet of the indoor water parks.
First of all, there is one park that, ahem, blows all of the others out of the water. The champion of the world -- timpani, please -- is Tropical Islands in Brand, Germany. Housed in a hangar structure designed for, of all things, a zeppelin, it spans over 710,000 square feet and is the largest freestanding building -- never mind water park -- in the world. It can accommodate up to 7000 visitors. Tropical Islands offers a variety of attractions and accommodations in addition to its indoor beach and water rides. Regardless, it easily qualifies as the world's largest indoor water park.
On the pages that follow, I wade through some other huge water parks. You can click through or jump directly to the topics below: