[Lower Deck]

...after a US Navy aircraft allegedly detected a 'heat stain' from 230ft below the surface, some 185miles from the coast...

I don't want to either get hopes up or dash them, but if this is 185 miles off shore then the water is a whole lot deeper than 230 feet.

[SunkenCiv]

What it means is, the sub is on the continental shelf, which is quite broad where they were heading.

http://www.orangesmile.com/ru/foto/oceans/antarctica.jpg