Could you please explain this line
Humans can swim for a certain time / distance. People can manage to swim the distance from England to France. The distance is within human capabilities.
Hi! This means that the distance across The Channel is so great (or long) that it’s the greatest distance that anyone could possibly swim.
If you look at the next line in the article, it compares The Channel to Mount Everest which is the ultimate challenge for anyone who climbs mountains. Likewise, The Channel is the ultimate challenge for anyone who likes to swim long distances because it’s so long.
If you need more explanation just let me know what words in particular are making this line confusing for you.
I hope that helps! 
I suspect (although I could be wrong) that the word “about” in that sentence is confusing. You can substitute “about” with “roughly” or “approximately”
The sentence would still work without “about” so instead it could say:
“The distance of the Channel is the furthest anyone (human) can swim in sea water.”
Which means anyone would struggle to swim a further distance than that distance in any sea.
x kilometers = the distance between France and England, by sea
x kilometers = the maximum distance that a human being can swim in the sea
And since x = x
The distance between Franc and England by sea equals the maximum distance that a human being can swim in the sea
or in other words
The distance across the Channel is about as far as it is possible for someone to swim in the sea
Where
The distance across the Channel = The distance between Franc and England by sea
and
the maximum distance that a human being can swim in the sea = as far as it is possible for someone to swim in the sea