Swimming is superb aerobic exercise (vigorous exercise that really pumps your heart and lungs) and very tiring. The two things are of course connected. You can swim further for longer by swimming more efficiently, which in turn means using as little energy as possible for each stroke by getting as much forward propulsion as possible.
With freestyle, for example, the object is to extend your hand as much as you can and bring it back as far as possible, dragging as much water back (with a cupped hand and bent forearm) as you possibly can. If you make a long, complete stroke with a proper follow-through, you are applying your pulling force for longer and each stroke will count for more.
To put it another way, if you want to produce the biggest possible change of momentum, you need to apply your force (pulling back on the water) far as long as possible…..with as long a stroke as possible and a good, complete follow-through. It is also worth remembering that the human body is a machine. Our limbs work like levers, pivoted at our joints and this multiplies force or speed. When you are doing freestyle, it is important to reach forward and pull your arm backward as much as you can. You get more leverage on the water that way and the force you create pulling backward will give you more force to go forward. A good follow-through also decelerates your limbs more slowly, and reducing the acceleration reduces the force they feel, reducing the likelihood of pulled muscles and other injuries.
It takes energy to push your body through water – and your body loses the same amount of energy in the process. The rate at which something uses energy is called power. According to an interesting article from Physicist Rhett Allain, champion swimmers can briefly achieve a power of 12oo watts (the maximum power of a clothes washing machine or a very powerful vacuum cleaner), which is about 3 times what a champion cyclist achieves. It’s not surprising that a swimmer uses more energy that a cyclist given that all 4 limbs of a swimmer are moving all the time (compared with just a cyclist’s legs), and in a more dense fluid. But it is surprising to me that the difference is so great! There seems to me no obvious reason why a human body working at full tilt should be able to produce so much more power in one fluid (water) that another (air).