Verified answer

Because the dolphin is approaching you, it's wavefronts will be reaching you more often than if it was stationary. So you will perceive a shorter wavelength and hence a higher frequency in accordance with the Doppler formula:

f = fs/(1 - vs/c)

The speed of sound in sea water is c = 1560 m/s (see link), much faster than in air.

So if f = 1150Hz, the actual frequency is

fs = 1150 * (1 - 3/1560) Hz = 1147.8 Hz

If the speed of sound in air of 340 m/s is (incorrectly) assumed the answer becomes:

fs = 1150 * (1 - 3/340) Hz = 1140 Hz