It is given that up direction be positive.
Ball velocity at the time is was dropped is 3 m/s.
g = -9.8 m/s²
s = -50 m
Now, using equation of motion for time :
s = ut + at²/2
3t - 4.9t² + 50 = 0
4.9t² - 3t - 50 = 0
Solving above equation, we get :
t = 3.52 s
Therefore, the rock will splash into the water after 3.52 s.