Answer:
The sea bed is 1125 m deep.
Step-by-step explanation:
Speed
It's the rate of change of the distance (d) traveled by an object over time (t).
The speed can be calculated by the formula:

It's known the speed of sound in water is v=1500 m/s. A sound wave is sent to the sea bed and bounces back to the ship 1.5 seconds later.
That is the time the wave takes to travel twice the distance from the ship to the sea bed, thus the required distance uses half of that time or t=1.5/2 = 0.75 seconds.
Solving the equation for d:
d = v.t
d = 1500 m/s * 0.75 s
d = 1125 m
The sea bed is 1125 m deep.