La vache boit de l'eau is drinking of the water. In English that would translate to some water. It could be any water, unknown water.
La vache boit l'eau is drinking the water. In English that is specific water. Water that is known to the speaker and the listener. eg: the water that has been put out for the cow to drink.
This post had included the user name of the person who posted the comment that I was responding to. He has since deleted his comment for personal reasons and has asked me to remove my reference to his user name which I have done.
I posted my comment before I saw yours because yours was hidden.
I guess your comment was hidden because someone disagrees with your take on the issue. Rather than post a correction, if one is available, they simply voted you off the island.
Anyway, I agree with your comment.
There is a difference between some water and the water in English. If this isn't the way that French expresses that difference then someone should post what is the correct way to express it. Not just vote other posters down.
L'eau is actually la eau because it is feminine.
Du is a contraction of de le which is the masculine form. If you used du you would be using the masculine form.
Because water is feminine you have to use the feminine form of prepositions, adjectives, articles etc to maintain agreement with the object.
In this case it is de la . La is liased with eau so it reads de l'eau.
Hope this helps.