Answer
The upward force of the water acting on the mattress is 806 N
Work Step by Step
We can think of the man and the mattress as a single system. Since the system is in equilibrium, the magnitude of the upward force $F_w$ of the water acting on the mattress is equal to the weight of the system:
$F_w = 806~N$
The upward force of the water acting on the mattress is 806 N.