I second what Bstanko6 has said about the hydrophone. It was an auxiliary tool and nothing more. The boat’s place was on the surface. There is much more reliance on the in-game hydro than was truly done IRL.
Check out the link to a guide I put together for Wolfpack. I made it to look like a KM publication but it isn’t, but all the methods in there are historical methods of gathering shooting data. With Hitman’s permission I added the great charts from his SH3 GUI too.
Note that the 9000 m plotting is due to the limits of the Wolfpack environment. In SH5 you can plot as the real boats did since you have hull-down ships - keep the mast tips just visible and estimate the range based on that by eye while overhauling.
http://www.subsim.com/radioroom/showthread.php?t=241480