Well, for Hazel and for a lot of people (and also a lot of places), water is both a creator and destroyer of life.
So let’s look at this from the perspective of a person, Hazel, and a place, Amsterdam.
Water makes life possible for Hazel, but the fluid in her lungs (which she refers to as water) is killing her.
Amsterdam would never have become a great city if it weren’t surrounded by water, but the city—which has benefited so much from its geography—is also drowning, and at constant risk of disaster from flooding.
I am of course not the first person to make this observation; the Latin phrase quod me nutrit, me destruit (that which nourishes me destroys me) goes way back. But I wanted to write a novel about the things that make life possible (and valuable) and how many of those things are also what makes life painful and temporary.
Water seemed like a good metaphor for getting into some of that stuff. (Plus water does all kinds of other convenient things, like follow the path of least resistance.) But you shouldn’t feel like you’re not doing a good job of reading the novel if you’re not conscious of that kind of stuff when you’re reading. There are many good ways to read a book, and if the metaphors work, you don’t need to be overly aware of them for them to move you and make you think.