Answer
The water exerts a force of $408~N$ on a person in the crowd.
Work Step by Step
A person's chest exerts an impulse on the water which is equal in magnitude to the water's change in momentum. Each second, the person's chest exerts a force which stops 24 kg of water moving at a rate of 17 m/s.
We can find the average force that the person's chest exerts on the water:
$F~t = mv$
$F = \frac{mv}{t}$
$F = \frac{(24~kg)(17~m/s)}{1.0~s}$
$F = 408~N$
Since a person exerts this force on the water, by Newton's third law, the water exerts a force of $408~N$ on a person in the crowd.
