Try to find a pool that slopes gradually, allowing you to stand in water up to your waist at one point, and your shoulders at another. Avoid pools shorter than 20 meters for lap swimming since they require frequent turns. A cap keeps hair out of your eyes, minimizes water resistance, insulates your head in cool water and protects your hair from chlorinated water. Gradually increase the number of laps you can do continuously. Aim to swim for at least 20 minutes without stopping.
Water walking or running is ideal for both aerobic benefit and resistance training.
Stand in waist-deep water to walk or run forwards and backwards. Treading water in a neck-deep depth for a sustained period is a good aerobic exercise. Pump your legs continuously as if you were on a bicycle. Repeatedly extend your arms, and then bend your elbows to cross them in front of your chest. For an aquatic version of a sit-up, for example, place your back against a corner of the pool, using your outstretched arms for support.