When Derek Rogers started Phoenix-based Aqua Therapy Tubs eight years ago, he didn’t know about walk-in bathtubs. When his friend and former business partner told him about how they were helping seniors, it hit home. “I used to help my grandfather bathe when he was alive and it wasn’t by choice, it was by necessity,” he says. “[Walk-in bathtubs] would’ve made all the difference.” Rogers’s grandfather was a disabled World War II veteran, and his...