Modern submarines are the best mobile platforms available for detecting sounds in the ocean. The whole front of a sub is a giant array of microphones coupled with major computing power all riding on a platform that itself is designed to be ultra-quite. AF447 included a French nuclear submarine in the initial search for the black boxes.
As SalNichols94807 commented, shallow water makes for a notoriously difficult acoustic environment. People familiar with the (wider) geographical area have stated on here that those waters are chock full of fishing vessels. Man made noise and natural noise in the water adds to the ambient noise, which will be much increased over that of an open ocean environment. Reverberations will be rife. There will be surface reflections and bottom bounce (depending what the bottom is made of). In deep water, sounds spread spherically, greatly dissipating the intensity. Shallow water, constrained by the surface and sea bed, causes the sound to spread cylindrically, meaning in a given volume the noise is greater. It's logarithmic and I can't be 4rsed digging out my old notes! 10Log versus 20Log.
That said, the shipping noise is low frequency whereas sonic locators on Flight Data Recorders is not. I hope the frequency will not be discussed. Ambient noise is still a factor.
In the Air France 447 case, the location was open ocean; much quieter acoustically and a submarine could manoeuvre in the deep water there. Submariners are notorious liars about where they are and where they've been but if they tell you they are loath to go anywhere a prang might ensue, I'd (guardedly) believe them. They don't like shallow water. I was about to quote Lance Corporal Jones but thought better!
They could use a surface vessel with a decent SONAR suite to listen for the location device. To cover any area AND listen is almost mutually exclusive. They may need to sprint and drift, otherwise their Own Ships Noise (OSN) may mask what they're listening for.
Otherwise, drop sonobuoys from fixed or rotary-winged aircraft. That might do it. I flew SAR on Air India 182 back in 1985. I was an acoustic specialist on my Nimrod Maritime Reconnaissance crew. We dropped buoys but heard nothing. The water there was over the 1000fathom line and was "quiet". Plus we dropped buoys at each of the 2 datums of wreckage so we were as near to the source as one could be, under the circumstances. We heard nothing. To be fair, sonic location devices then were in their infancy and I've no idea if the Air India 182 aircraft had been fitted out with such a device.
Other commentators here have questioned whether there COULD be wreckage and/or oil/fuel. All I can add is what I saw: In the case of the Air India182, each of the 2 datums (60 nautical miles apart) had an oil slick as a huge "lead-in" feature. At each datum there was a large collection of jumble; all that could float. That's all I'll say on that matter.
I used the word "oil", in all probability for convenience. Some people might nit-pick but such fluids from an aircraft wreck will come from a variety of hydrocarbon, mineral and synthetic sources. On that day, on the surface of the ocean, there was a very obviously "oily" film. The sea was calm. In fact it was a beautiful day and those facts messed with our heads in looking at what we were seeing!
There might be major differences in this particular sad case, today. And so, for what it's worth, I offer these recollections in the hope that they might help!