"...the submarine Thresher lost two months earlier with all hands and how it must have been in the last doomed minutes..."
I can't help myself, this sentence always reminds me of Jack's comment about the boneless blue at their last trip:
"...but the boneless blue was so deep, said Jack, that he might drown looking up."
This is just an inversion of the sinking submarine: the endless blues sky is just the same as the endless blue sea, only above instead of beneath. And instead of sinking down into the blue of the ocean, it's drowning while looking up.
The sentence about the bonless blue of course leads my thought to the next two sentences:
"...he had drowned in his own blood."
"...blood choking down Jack's throat and nobody to turn him over."
It's all about drowning. Gawd, this is so depressing stuff

. I think it is such a cruel, cruel way to die, unconsious or not.