I can relate very well on feeling the need for preciscion. Been there, done that ... but thankfully its wearing off. It's a hard pill to swallow but worrying about it just takes too long, and too much stress wich doesn't get you much further. But don't let me stop you trying to attain it.
As for the picture. Those length numbers are not going change due to a mod. They are hardcoded. Yes it shows 100m but it also shows that when it is accros upto 3 gridsquares of 50m wide. In other words it shows 0.1 between 0.05km and 0.15km. The number flips over nicely at the 50m pixel due to rounding off.
For hydrophone use go to the station yourself with the 'H' key. There is a button on it that ups the volume a bit. Just click it when the tooltip shows. You'll have to move the needle yourself. You can hear sounds that are alot further away than your crew does. You can hear upto about 34km radius. (crew only 18km or 20km or so) If you want to know the correct bearing listen to the sides where the sound is barely audible and use the middle of it. It's too difficult to determine where the peak volume is. But you'll find out you can still be sometimes 1 or 2 degrees wrong.
As for technique:
http://www.filefront.com/14186971/Hy...on.part01.rar/
(for some reason I keep loosing the thread about this movie, you'll have to search for it)