I know that in Spanish, they often use the word "the" when we wouldn't. For example, we say, "That's life," when they say, "Eso es la vida." In Esperanto, they seem to say "La angla" when we just say English. How do you know whether they're talking about water in general, or specifically THE water?
It depends on the context - sometimes la is used in Esperanto when the would not be used in English.
However, in this instance, both English and Esperanto require the definite article, the/la, and "the water is not red" and "water is not red" are not synonymous.
"Water is not red" is general. Water, typically, is not red. This is just a general statement of fact.
"The water is not red" is specific. Now we are referring to specific water that is not red.
Both sentences are correct English, but they don't mean the same as each other, and only one of them is a correct translation of the Esperanto phrase.