This is true. Splash/TBA is the only ride at Disneyland where you get wet purposely. And Disneyland is one of the most visited and crowded theme parks on the planet. Disneyland will be crowded, SoCal is often warm, thus TBA will have a long line.
Disneyland: 18 Million annual attendance, 1 water ride (TBA)
Knott's Berry Farm: 4 Million annual attendance, 2 water rides (Timber Mt. Log Ride, Calico Rapids)
Sea World: 4 Million annual attendance, 2 water rides (Atlantis, Shipwreck Rapids)
Magic Mountain: 3 Million annual attendance, 2 water rides (Jet Stream, Roaring Rapids)
Honestly, they could have rethemed Splash Mountain into a trip through a dentist's office, and it would still have a long line.