“As others have pointed out, the Gulf is shallow, and relatively small. Any sounds produced by boats and other vessels radiate and bounce in every direction.”
Yes, that is exactly what I said!
You did, but you also said "a good sonarman would've heard it [the fishing boat]."
I'm saying, perhaps, a good sonarman would've heard it. The conditions are challenging in the Gulf, as we both agree. The fishing boat might've been idling, DIW, or otherwise indistinguishable from the ambient noise, in which case, even the best sonarmen would have a hard time detecting it.
I'm not excusing the incident, I'm just intimating there may be circumstances out of the control of sonarmen or the captain.