[GATOR NAVY]

...fathometer readings can be inaccurate...

The operative word here is "can". The author doesn't say the soundings will always be accurate or that they will never be inaccurate. I don't know what kind of fathometer sunmarines have, but I know that I am doubtful of the ability of my fathometer to consistantly track at 30 kts with a water depth of 1000 fathoms. -USN surface navigator

[Light Speed]

Navy to chart ocean floor after tsunamiNavy cartographers say navigation through the Indian Ocean will be problematic for years due to last month’s violent earthquake and deadly tsunami, both of which altered normal shipping routes.
Old charts may have caused fatal accident of U.S. nuclear submarine
Seafloor Still About 90 Percent Unknown, Experts Say

[Doohickey]

It's a BQN-17. Does that help?