Summarize:

The type of swimming you do to stay fit will depend on what you want to accomplish with your fitness routine. For most fitness swimmers, a combination of strokes is used to provide a full-body workout.  For example, if you want to lose weight, you'll want to focus on the strokes that you can perform well and for the longest period of time, so you can get the greatest fat-burning benefits out of your exercise sessions. Once you've identified your major goal or goals, break them down into small, reachable goals so you can constantly monitor your progress and get a continual sense of achievement. Once you've gotten used to swimming regularly, you'll get more out of each swimming session if you follow a specific plan rather than simply lapping up and down the pool without focus.  Check swimming sites online, such as the U.S. Masters Swimming Program at usms.org, or the England Masters at swimming.org. You'll find various swimming plans tailored to achieving particular fitness goals, as well as tips on how to improve the efficiency of your stroke. Other fitness clubs or local swimming clubs may also publish swimming workout routines on their websites. Some pools also provide suggested workout programs on laminated sheets along the walls of the pool area or locker room. Particularly if you're just getting started with swimming, you may benefit from the structure of a class and an instructor who can correct your form. There are a wide variety of classes available if you want to swim to stay fit.   If you find the idea of swimming laps boring, you might enjoy a water aerobics class, or a different water exercise class. You also might choose different classes to meet your specific fitness goals. For example, if you want to improve your flexibility, you might try a water yoga class. You'll only see overall fitness benefits if you swim on a regular basis. Ideally, try to swim at least two or three times a week. Even if you can only swim once a week, you'll still improve your fitness provided you're keeping active in other ways.  If you've just started, even those first few laps may prove more difficult than you anticipated. Just keep with it. Aim to increase the amount of time you swim with each visit. For example, your first few swims you may only be able to swim for 10 minutes. After a few 10-minute swims, increase your time to 15 minutes. The next week, increase your time again until you've built up to 45- or 60-minute swims. Even if you're a proficient swimmer, you should include other types of exercise besides just swimming laps. If you want to keep your workout in the water, consider taking a water aerobics class.   Variety is important when swimming laps as well. Sticking to a single stroke can get boring after awhile, which can make it difficult to stay motivated and stick with your fitness routine. It also can be beneficial to supplement your swimming workouts with strength training workouts to help build your muscles. Focus on exercises that build the muscles you use the most when swimming, such as your shoulders, arms, and legs. Rather than going it alone, getting a swim coach or swimming with a friend or family member can help you stay motivated. If you enjoy a little friendly competition, you can race each other or compete for most laps or fastest overall time.  You also may be less inclined to skip swimming sessions if there's another person involved who is counting on you to show up and help keep you honest. Before you jump into the pool, spend a little time warming up your arms and legs. Do repetitive movements that utilize the full range of movements in your arms and legs for five to ten minutes. As with any exercise, you want to start your swimming session with a warm-up and end it with a cool-down to give your body a chance to readjust to its pre-exercise state.  With a swimming workout, a few slow laps at the beginning and end of your session function perfectly well as a warm-up or a cool-down. You also may want to treat yourself to a hot tub or sauna if those are available where you swim. If your pool has a café, a healthy snack or protein shake after your swim can be invigorating and rewarding.
Set your fitness goals. Search online for workout plans. Join a class. Swim consistently. Vary your routine. Join forces with a buddy. Do a dry-land warm up before you swim. Include a cool-down at the end of each swim.