You have probably learned that when you want to count nouns in Chinese, a classifier, or measure word, is needed, for instance, one book 一本書, two roads 兩條路, three dogs 三隻狗 ... etc. 瓶子 as a noun, meaning "bottle", needs its own classifier 个.
What do you do when what you're trying to count isn't countable, like a liquid or a gas? In such cases, you will need a container-measure. 瓶 is one of these container-measures. When it's used as a measure or classifier, it's no longer a noun and does not need a classifier as such. That is why 一瓶水 is correct but *一个瓶水 is not.
Another example of a container-measures would be 杯 (cup/glass).
一個杯子 a cup/glass
一杯水 a cup/glass of water
There are other kinds of classifiers as well. A common plural classifier is 些. And classifiers are also needed for abstract things such as thoughts, love affairs ... etc. But I will leave those for another question on another day.