If you know French could you help? Please?

Okay… I’ve been learning French for some time and I know my basic grammar and whatnot. However, there is one thing which has always never failed to baffle me. It’s regarding the noun for water: “eau”

I know that it’s a feminine noun, which would mean that you’d have things like “la pollution de l’eau” (water pollution) or “la fête de l’eau” (celebration of water) because it’s “de la” and not “du”.

However, why is “a bottle of water” known as “une bouteille d’eau” and not “une bouteille de l’eau”? It doesn’t seem to make any sense to me. I’ve been searching on the net and haven’t been able to come up with any straight answers to this. Is this an exception to the rule or am I missing something here? And is this present in any other cases in this otherwise beautiful language?