Answer
Rob is 9.81 meters below the boat.
Work Step by Step
We can find the time it takes the sound to travel 5.00 meters through the air:
$t = \frac{d}{v} = \frac{5.00~m}{343~m/s} = 0.0146~s$
Since the total time is $0.0210~s$, the time that the sound traveled in the water is $0.0210~s-0.0146~s$ which is $0.0064~s$. We can find the distance that the sound wave traveled in the water:
$d = v~t = (1533~m/s)(0.0064~s) = 9.81~m$
Rob is 9.81 meters below the boat.
