Swimming requires muscular endurance. As a survival skill or an aerobic activity, it demands sufficient cardiovascular development to ensure increased oxygen-delivery to all the working muscles throughout the swimmer's body. Thus, as a swimmer, your cardiovascular capacity informs your overall power-output, and endurance is the working essence of that physiological interplay. It is an athletic trait that swimming both demands and develops.
Endurance Versus Power: What's The Difference?
Answer Fitness provides a clear distinction: "Muscular endurance is the ability of a muscle or muscle group to do repeated contractions against a less-than-maximum resistance for a given period of time. This is in contrast to muscular strength, which is the greatest amount of force that a muscle or muscle group can exert in a single effort." This helps to explain why swimming is predominantly endurance-related. Muscle strength determines the maximum force behind your stroke, but swimming is highly technical and makes use of the physics of water flow and energy efficiency rather than brute power alone. You don't need a weight-lifter's muscles to excel at swimming.
Water: Constant Resistance
Water has power despite its "gentle" appearance. Unlike other sports environments, water is always moving around and against your body, requiring that you master skills such as horizontal body-orientation, constant self-stabilization, timed breathing and an ability to exert muscular force in all directions. Water applies incessant pressure on your body--and you have to push right back, without resting. This what gives swimming its historic reputation as an endurance sport; without endurance, swimmers ultimately will cease all forward motion.
Timed-Breathing Supports Endurance
A good coach can help you to identify optimal breathing patterns and rhythms. Even professional swimmers' endurance suffers if their breathing is not executed correctly; breath control allows efficient oxygen use, and muscles will respond to increased or decreased oxygen levels accordingly. Coaches can observe your strokes and help to identify wasted motions that lead to inefficient oxygen use and muscular fatigue.
Distance Plus Some Intensity
Swimming is a distance sport. Sprint-distances in professional racing are a full 50 yards rather than a mere five or 10, and open-water races such as the Swim Around Manhattan in New York City can be as long as 24 miles. Speaking of open water, should you ever unfortunately be capsized at sea, your ability to swim and tread water for extended periods of time--minutes, possibly hours--is going to determine your survival, so endurance is literally a life-saving aspect of overall fitness. But don't just swim long, slow yardage; mix some fast, intense sequences into your workouts, because intense intervals of one to two minutes enhances your body's overall ability to use oxygen and perform for long periods of time.
Training and Resistance Swimming
You can develop endurance even if you do not have access to an Olympic sized pool. In a small pool or water-area, you can try "resistance swimming," which utilizes either a fixed current-machine similar to a hot tub's water-jets, or a set of thick elastic bands tethering you to the pool edge so that you can swim against the elastic's resistance. Search Your Love: Tethered Swimming has ideal descriptions of resistance training and opens up the possibility of endless swimming--or simply swimming to your heart's content.