Fish swim with no discernible effort. Conservation of momentum. With only the slightest tail flick, fish can glide along for many body lengths with little apparent deceleration. Human swimmers must stroke and kick ceaselessly at even the slowest speeds. 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.