Exercising in water is a different environment than on land because of the water's buoyancy. Water helps you float to take weight off your joints, but it also provides more resistance than air. When you move your body through a pool, the water presses on your body from every angle. You can use this resistance to strengthen your arms either with or without water weights.
You have the option to exercise your arms in a pool without using any equipment. An example is the step star. This exercise is similar to plank pose and a side plank in yoga. To perform this exercise, face the stairs of the pool with your feet on the bottom. Lean forward and place your palms on the pool step with your arms straight as if you were about to do a pushup. The water should come up to your elbows, but no higher. Raise your right arm perpendicular to your body as you twist to the side. Your body forms a "T," with your right arm pointing at the sky and your legs stacked. Lift your right leg straight to the sky too until it is parallel to the pool bottom. Switch sides.
You can modify Pilates exercises that work your arms for use in water. An example is the 50:50. This standing Pilates exercise targets your triceps and shoulders to tone your arms. It also works your abs, lower back, chest, back, inner thighs, outer thighs and hamstrings. To perform a 50:50, stand in the pool submerged to your chest with your arms straight at your sides, palms facing backward. Extend your right leg straight backward and point your toes so they are approximately 6 to 12 inches from the pool bottom. Balance on your left leg and pump your arms forward and backward quickly for 50 seconds. Move your arms only a few inches and keep them straight. Switch legs.
Arm exercises you do on land can be brought into the pool using foam water dumbbells. These dumbbells work against the resistance of the pool to tone your muscles. For example, a triceps kickback exercise uses a single dumbbell to target the triceps of one arm. You can stand with your body submerged to your sternum and use a water dumbbell to perform kickbacks in a pool. To begin, stand holding a dumbbell in your right hand with your arm bent and your elbow touching your side. Turn your palm to face inward. Step your left foot forward and rest your left hand on your thigh, bending forward slightly from the waist. Then, extend your right arm straight behind you. Bend you arm back toward the front of your shoulder to complete a triceps kickback on your right arm. Repeat on the left side.
You can also do barbell exercises for your arms in a pool using a water barbell. The water barbell works the same way as the dumbbells, using the resistance of the water on the weight to strengthen your muscles. The standing barbell curl is a water barbell exercise for the biceps. The movement for this exercise is the same as a standing curl on land, but in the water you want to make sure you are submerged up to your chest so the barbell stays underwater. To begin, hold the water barbell with your arms straight toward the bottom of the pool and your palms facing forward, hands shoulder-width apart. Raise the barbell toward your chest by flexing your elbows.