Fish maintain a constant sleek and tapered silhouette. Humans move through a constantly changing series of awkward and blocky shapes, mainly caused by the head and limbs. Fish leave barely a ripple in their wake no matter how fast they swim. Humans churn up the water crazily, leaving a cascade of bubbles with every stroke or kick. And any feeble attempt at speed causes turbulence to increase exponentially.
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.