A "good" sonarman probably hears every boat in the Persian Gulf--AT THE SAME TIME! 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. And to your statement, fishing boats make just as much noise, depending on their size. Non-military vessels do not put much emphasis on sound-proofing. Furthermore, I couldn't tell from the story where the fishing boat was, if it was moving, or dead in the water. Its first detection may have been only through the periscope. Bad luck, as much as anything else.
“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!