Question 37199
Assuming that his travel was in a straight line, which it probably wasn't, and assuming that the current was constant throughout his swim, which it wasn't, the problem becomes a simple Pythagorean theorem problem.  
You need to find the hypotenuse of a right triangle 400 m on one leg and 100 m on the other.
So c^2 = a^2 + b^2 = 400^2 + 100^2 = 170000
And c = square root (170000) or about 412.3 m