Also, it’s very difficult to keep secret a project that involves more than 10 people, especially if it involves building things.
But not impossible, and there are a lot of examples of things being built (or destroyed for that matter) in perfect secrecy involving groups larger than 10 people.
People retire or fall out, documents get lost, someone slips somewhere. It does not take much for the existence of a project to become public knowledge. It’s the other side of the OPSEC coin: a single mistake and some information gets available.
Still, I am not saying this is impossible, merely improbable. Which, combined with the other factors, makes the story being a secret military project implausible.
Could be communication but why would you use a medium as terrible and lossy as sound in water?