a cola 是口语说法，相当于a tin of cola，一罐或一听可乐的意思。而water 是不可数名词，也是一个泛指的词，所以没有这种用法。

There was a cola by the side of the house.

房子旁边有一棵可乐果树。

The man got on the taxi after he bought a Cola.

那个人买了一杯可乐以后上了公交车。

You got me a cola drink?

你们送我一瓶可乐？