This is a lighter version of triceps dips, performed in the water with easier resistance, namely the noodle, as opposed to lifting your body weight on dryland. Although relatively easier, triceps dips with noodle can create enough challenging resistance to shape your triceps.
Punches make great arm exercises in the pool. They can also make effective aerobic workouts if performed rapidly for longer time. However, combining punches with upper body twisting adds a lot to the exercise.
While the main focus is arm muscles, this exercise also targets your core, and increases your flexibility. It's highly recommended to perform with aqua dumbbells or any other resistance.
A great way to strengthen your shoulder (internal and external rotation) muscles in a single exercise. It is best performed in the shallow-end. You will need aqua dumbbells or other water resistance devices. For recovering from an injury, perform without dumbbells.