Given that there're navy members in the background, it's more likely to be Kongou's actual sinking.
I mean, unless they bring normal people out to the open sea to battle against abyssals, that is.
She's probably mentally re enacting kongou's original sinking in '44, Hamakaze rescued Kongou's survivors so shes just adding a mental image of her ship girl self to the event. Those men in the background are likely Kongou's and her original crew.
thats how I interpret it at least.
ForTo have that feelingthe second timeHamakaze? What's the matter?Nothing...Nothing at all!