> One by one your neurons are replaced by water. At which neuron are you no longer you?

You have described an aspect of ageing & decline.

> doesn't imply that there is no such change

I made no such assertion, obviously something is changing. The question is whether or not that change matters to your core identity. The thought experiment in my opinion shows that as long as continuity is maintained and as long as you are happy with the results you remain yourself and gained something in the process.