Getting back on topic...
Sound in the atmosphere doesn't couple well into the ocean. That just means that you can't hear things in the sky very well whilst under water. But if it's a loud enough aircraft, it will be detectable. I've no idea how loud it'd have to be though.
It's the same physics behind the reason an ultrasound scan needs a gel between the scanner probe and the missus' belly. Without it the sound energy coming from the probe would have to travel in part through the air (unless the operator presses really hard). The gel ensures that there's a good impedance match between the probe and the belly. Same for the physio's ultrasound massager. Apparently those used to break quite easily if switched on without being gelled up and in contact with skin. The impedance mismatch reflects a lot of power back into the probe, and it cooks itself and no one's the wiser...