Humans are clearly not as well suited for the water as we are for land, just look at a human body. We can't hold our breath for very long, and we get very tired very quickly in water. The main reason for this is the difference between the water and the air. The biggest difference between the two is that water is much more dense. When a human walks on land, the main thing your body has to do is work against gravity and then friction between our feet and the floor. Air resistance also becomes a bigger factor as wind increases or we pick up speed. The main force opposing a human in water is drag or water resistance. This can be controlled though by the way we move through the water. The same way a cyclist tries to minimize surface area being presented to the wind, a swimmer wants to minimize the resistance to water they may be facing. This is why many of the different ways of swimming are so efficient. The different strokes used in competitive swimming and just to swim fast, are ways humans have found to effectively cut down on water resistance and drag.