To find the depth of the ocean using the time it takes for the echo to return, we can use the formula:

\(\text{Distance} = \text{Speed} \times \text{Time}\)

The time given (2.5 seconds) is the total time for the sound to travel to the ocean floor and back up to the surface. Therefore, the time for the sound to reach the bottom is half of the total time:

\(\text{Time to reach the bottom} = \frac{2.5 \, \text{s}}{2} = 1.25 \, \text{s}\)

Using the speed of sound in water, which is \(1,400 \, \text{m/s}\), we can calculate the depth \(d\):

\(d = \text{Speed} \times \text{Time to reach the bottom}\)

Substituting the values:

\(d = 1,400 \, \text{m/s} \times 1.25 \, \text{s}\)

\(d = 1,400 \times 1.25 = 1,750 \, \text{m}\)

The depth of the ocean is:\(\boxed{1,750 \, \text{m}}\)