A substantially hollow exercise and rehabilitation device is disclosed, and is useable by athletes and others in training and/or rehabilitation sessions to develop stronger and more precise movements. A user typically stands in a pool of water approximately chest-deep and moves the device through the water in an approximation of a proper swing for a sport, or in a motion designed to rehabilitate or strengthen one or more muscles or muscle groups. The device includes holes that tend to minimize turbulence that a solid device would generate, yet provides both resistance and good haptic feedback to the user. The holes are adjustable in size to provide variable resistance. These features develop or redevelop strength and consistency in the user's movement. Analogous training benefits are obtained for many activities, such as batting, golfing, and tennis, and for rehabilitation and strength training using devices in the shape of tools and other instruments.