"There is a lot of water."
Basically "wa" is used when the people in the conversation know and expect what the subject is. "Ga" is used when they don't know what the subject is going to be.
Who are you? (Anata wa dare desu ka?) Watashi wa Lloyd desu. The person asking the question already knows that the subject is the person they're talking to, so "Lloyd" answers with "wa'.
Who is Lloyd? (Lloyd wa dare desu ka?) Watashi ga Lloyd desu. The person asking the question doesn't know "who" the answer is going to be, so they answer with "ga". The person asking the question knows what the subject is (Lloyd) so they use "wa".
If you were to translate these answers, the first one would be "I am Lloyd" and the second would (very roughly) translate to "The person named Lloyd is I".
In this case, "Mizu ga takusan ari masu". The sentence doesn't know that the subject (user) knows what the subject is, so they use "ga". "Ga" is usually used when defining the subject of the sentence/situation. It's mainly built on context.
Wa -Marks the topic of a sentence. -Contrasts one option or thing against others. -Used when elaborating on things already known to the listener.
Ga -Marks the subject of a verb. -Separates/calls out one option or thing from others. -Used when adding new things/information to a conversation. -Used when describing specific things with adjectives. -Marks objects of potential-form verbs. (can/able to) -Connects clauses with a meaning of "but" or "despite".
Since Japanese is such a different language compared to English, there are many structural and grammatical differences. I know these because I know how to pronounce/speak Japanese and the sentence structures, but I'm relearning how to read it. It would be nice for a lot of people to somehow learn the syntax of Japanese and how different sentences are structured.
For example, the differences between "wa" and "ga", many different ways to read a specific kanji based on the context, and others. For example, there are many different ways to say "I" in Japanese, but most textbooks teach by saying "watashi". That's not necessarily wrong, but there are many other ways of saying "I" based on the context. Guys can say "Ore" or "Boku", while girls (and some guys) say "watashi". There are also different levels of politeness in these different ways of saying "I". You wouldn't say "Ore" when talking to your boss or someone higher up than you.
Hopefully that helped a little bit, Danmoller.