Method of propulsion. Fish propel themselves with effortless, unhurried undulation or oscillation of the core-body. Viewed from underwater, it is clearly a massively inefficient way to propel a body through water, compared to fishlike propulsion. Just as clearly it is also the source of all the bubbles, turbulence, and non-slippery body shapes. Fish are ideally designed from eons of evolution to minimize drag as they swim.
Humans are almost as ideally designed to maximize it.
Well, I'll encourage you to think about it first, last, and always, so here's a quick primer on how to it. Why Water Is a Wall First keep in mind that water is nearly 1,000 times more dense than air. In a medium as dense as water, it takes very little speed to create a whole lot of drag. When anything tries to move through water, water pushes back. In cycling, your efficiency is increased by gearing.