Swimming stars : dolphins vs humans

From Richard Hammond's Invisible Worlds

In the high speed world, water becomes thick, dense - an alien environment that we struggle to get through.

Over time a strong swimmer can manage about 2 miles an hour.
But that’s nothing compared to other mammals like the dolphin, that can reach speeds 5 times faster.
But it's only in slow motion that we can see why the dolphin is so much at home here, and why we aren't.
Water is 800 times thicker than air - so thick that it pulls and distorts our soft bodies as we swim, causing drag which holds us back.
The water doesn't distort the dolphin's body at all, passing smoothly over it.
The dolphin has much thicker skin, like rubber, that stops his blubber from flapping.

So while we struggle to make headway, the dolphin is barely trying, and when they need to go faster, they can make it look effortless.

Using state-of-the-art technologies, Richard Hammond goes beyond the limits of the naked eye and explores the hidden secrets of the invisible world around us.

Human vision is pretty miraculous, but our eyes aren’t quite as powerful as you might imagine. What we can see is stuff that reflects or emits light with wavelengths in a very narrow band (since you ask, from about 750 to 400 nanometres).
What we can’t see is the rest.
That’s all matter that reflects or emits light over the other 99.99% percent of the spectrum.
In fact, we’re almost blind.