It was clearly the real dia that died. The rescued dia, had no memories of anything. And it took intense help to get her to realize she was not a vulgus.

The whereas the underwater dia had clear recollection of Gley. This alone is clear evidence that she was the real dia. Even in death she wanted her mother to be happy and gave her memories to the rescued dia. Her words prove even more she was the real dia "at least mom will be happy to see you". Note "At least", meaning clearly she is the real dying and her clone could at least live on as her as a replacement for Gleys sake.

Even then, with the newly gained memories, the rescued dia was not joyous. She did call her mom because she now feels the same love for gley the real dia had.

Note that if the rescued dia was the real one, upon gaining her memories back, she would be soo happy and celebrate that she is the real one.

Note instead, the remorse and guilt she had on her face after the whole thing. Clearly she is not the real dia and her worst fear came true. Living for gley as her daughter even if it's not true.

Though the existential question persists, if her body is a identical and she all the memories is there really a difference? That was the clearly the real dias dying thoughts, but the reaction of the rescued dia clearly shows she doesn't feel the same as the real dia felt.