LGear
Secret Society BLANKET
Only none of the ships during the test had any crew aboard. They weren't ships, not really. They were just hulls that had at one point been ships. A ship needs a hull and a soul. Without her crew, without her soul, she's just so much iron floating on a pond. Some of their crew does rub off on the ship, which is how you get shipgirls. But a ship without her crew just isn't whole.
A ship "dies" when the last of her crew steps off her for the last time. Going to the breakers is less a torturous death, and more the embalmers doing their work. The ships at crossroads were generally aware of what was happening to them, but they were already well on their way to the locker before the first bomb dropped. What pain they felt was dulled almost to nothing, and the last thing Sara felt was a vague sense of pride that she'd found one last way to serve her country.
Plus, shipgirls have very different sensitivities to pain, which will come in handy for a certain someone...
This particular interpretation of shipgirls seems to have some basis in canon: none of the Crossroads survivors ingame have lines that have them vividly remember the nukes, just some vague, fleeting memories of it. For example, Nagato:
"What is it I wonder? What is that landscape, deep inside my memories? Friendly and enemy ships, and then that intense light... Hey, Admiral...I must be stressed out."
Here's another from Prinz Eugen:
"It's 9am. Hm? Ah, it's Nagato! Ooy, Na-gato! ...hm? Where have we met? That is, of course! ...uh... huh? Umm... where was it...?"
She knows Nagato, but doesn't remember where they first met.