Aircraft can drop passive sonobouys which will detect the locator beacon.
The P3 carries about 100 of them in rotary dispensers. They can be set for depth and 2, 4 and 8 hour endurance but, in this scenario where you are looking for a short range transmission, you would set them for the shortest life so as not to block up the transmission channels to the aircraft.
Also temperature, salinity and thermal profile of the water will greatly decrease or increase the distance at which an acoustic beacon can be detected, you just need to search both above and below the isothermal layer.
Hope that helps.