TL,DR: Don't use 을/를 with 있다.

Before 있다, your "English" object becomes a subject in Korean (with 이/가). I used to think of it as "the water is/exists/...

Maybe think of it as a slightly different verb than the English "to have" with different grammar than in English.

Now, this of course makes it weird once you want to add an actual subject to the sentence, like is done here. You'd end up with two subjects in a sense.

So, that's where the subject here ends up using the topic particle (은/는) to avoid having two words with the 이 particle.

Disclaimer: I'm not a native speaker. I was never taught this particular grammar point. I just deduced it from frequent usage. I may be completely wrong on this. Take it with a grain of salt :)