5" Mini Hydro Ball – Precision Training for Throwers Meet the 5" Mini Hydro Ball, the ultimate training tool designed with throwers in mind. This compact, water-filled powerhouse provides a dynamic, ever-shifting load to elevate your strength, stability, and throwing power. Modeled after our popular 6.5-inch hydro ball, this mini version is perfect for building the control, improving grip strength, shoulder mobility and more. We wanted to create a ball that younger athletes can have a better grip on.