Exercising in deep water gives you a highly efficient aqua-aerobic workout. This is because the majority of your body is submerged -- when more of you has to move through water, more resistance is encountered. You also tend to be more buoyant, which keeps you constantly fighting the urge to float. Bonus: You can stay cool in the pool while getting a high-intensity, low-impact workout.
Water that's shoulder- or neck-deep can make you feel like you've run a mile by the time you make it to the other side of the pool. The powerful resistance of the water helps increase your cardiovascular burn when doing exercises normally reserved for land. Start with walking, swinging your arms under the water and lifting your knees high. When running, lean forward slightly to help push through the water's resistance. You can also try jumping jacks, raising your arms as high as you can without breaking the surface of the water for maximum resistance -- jump high and land with your legs spread, then jump high again to close them. Wearing a flotation belt can help keep you upright as you move through the deep water. Lift your legs off the pool floor and tread water, changing it up by moving only your legs for a few seconds then only your arms. Feel that heart pumping?
You need deep water to work your arms effectively because they must stay underwater to take advantage of the resistance. Punch your arms forward, out from your chest, or lift and lower them in front of you and out to the sides. Use water dumbbells for more resistance, or try a beach ball. Feather-light on land, beach balls require strength to push beneath the water. Try pushing one down toward your thighs, letting your upper body rise out of the water slightly to help force the ball completely under water. Have fun with this one.
To work your core, stick by the side of the pool in the deep end where your feet can't touch. Put your back against the side and hold onto it behind you. Keep your legs straight and lift them higher than waist level if you can. Hold them there for a couple of seconds, then lower again. These also work your thigh muscles. Do this exercise with your knees bent as well, bringing them up toward your chest together. Get rid of the sides of your muffin top by swinging your legs side to side at waist level to work your oblique muscles in the side of your stomach. For more resistance, hold a noodle between your knees as you workout. Don't reward yourself afterward with real muffins and noodles, though. You don't want all your work to be for nothing!
While still hanging on to the side of the pool, lift your legs to waist level and open and close them to target the difficult adductors and abductors in your inner and outer thighs. Stand on the bottom and lift up on your toes, then lower back down until your feet are flat on the bottom to work your calves. This works best when you're in water as deep as possible, such as neck height, to provide the most resistance.