To maintain your fitness level, you must be consistent. Rather than participating in extreme exercise and dieting occasionally, you should develop the habit of practicing a moderate exercise and diet plan daily, on an ongoing basis. When you get 15 minutes to half an hour of light to moderate exercise daily and eat a balanced, healthful diet, you will gain and maintain the best in fitness.
Keep track of what you have been doing, and what is working for you. Evaluate what exercises you enjoy and what left you bored. A fitness plan should never be stagnant. Keep switching things up and changing things that aren't working for you. A fitness plan that is set in stone is a fitness plan you'll stop using.
Before plunging right in the water and starting to swim, spend some time ducking your body, head included, in and out of the water. Breathe in while you're on the surface and breathe out while you are under. This gives you time to acclimatize to the water and your swimming will enjoy the benefits.