My understanding is that underwater sounds can reflect back off the bottom and / or thermal layers in unpredictable ways. There are tales of submarines being tracked from more than a thousand miles away under the right acoustic conditions. I have no problem imagining that a pinger's signal could turn up three hundred miles away.
No, because the signal from the emergency beacon pinger is VERY DIFFERENT from those emmited by ships, submarines and singing whales, that can be heard hundred of miles away.
The sounds that can be heard hundreds of miles away - because of phenomena called ducting - are low frequency sounds, that can travel in water great distances with minimal loss of energy. Such sounds can be detecte from long distances in similar way like astronomers detect very weak light of distant stars or galactics - by using very large reflector and focusing the energy from large area to very small area (for example using long towed sonars or finding a natural sound convergence zone).
The emergency pinger generates a high frequency sound, which is highly attenuated in water, and it loses it's energy relatively quickly with distance.
So at distance hundred of miles it can't be detected even using a large reflector or antenna or sound convergence zone - because there is nothing to detect.
An analogy - a large astronomical telescope can be used to detect very, very weak energy from a distant star (too weak to be registered by a CCD detector). The light is really very, very weak but IT IS THERE, only "very diluted". If you are smart enough, you can build a device that would amplify it to detectable level. So you build a telescope with a big mirror, collect the weak energy from large area and focus it on a small area - so it's again detectable by your CCD detector.
Trying to detect 37.5kHz pinger from 100nm by any sensor, would be like trying to use an astronomical telescope to detect a light from a street lamp that is only 10nm away - but in a thick fog. The telescope is big, but it won'd detect a light from the lamp because it was attenuated by the fog to practically zero level and there is simply no light that can be focused, period.
It doesn't matter how big your telescope/towed array is, if you are 100nm away you can't amplify a signal that was attenuated to zero level before it reached 10nm. It doesn't matter that there is a "natural water telescope" named convergence zone - it would amplify sounds and you would hear even wery weak signals of low or medium frequency, but you won't hear an ultrasonic pinger because at such distance it;s sound energy is not "diluted" - it doesn't exist anymore.