A man who can  swim at a speed v reltive to the water wants to cross a river of width of flowing with a speed $u$ . The point opposite him across the river is A

A man can swim along OA if he goes along OB with velocity v . The velocity of man along
 $OA=\sqrt{v^2-u^2}$. Time taken to go from  O  to  $A=\frac{d}{\sqrt{v^2-u^2}}$
The minimum time in which he can cross the river is if he goes along OA with velocity v . This time $=\frac{d}{v}$ 
If $u>v,$  then the time taken to go from O  to A  is imaginary, so  the man cannot cross the river.