Does it have a less literal meaning that "nothing left behind"?
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The comment section basically has all the answer, so just to summarize that,
後はない in 「水だな. 後は無い.」 means "other than that, nothing". To better understand this, mentally picture a list where 水 is at the top, and then go through the list from top to bottom. This expression is saying that the list actually only contains one item that is 水, and so you say "after 水, there is nothing [on the list]".
後がない is a bit more idiomatic expression that emphasizes a desprate situation / the last stand / the final chance to make or break, such as この試合に負けるともう後がない (if they lose this game, that's no hope [of winning a league]) or この試験に落ちるともう後がない (if I don't make this exam, I'll be kicked out from the school.)
In a way, 後がない refers to a situation similar to 後はない, as in "aside from this one thing, there's no more in the list", but 後がない really emphasizes desparation. Perhaps because of that, personally I don't use 後はない in writing, and probably not much in spoken Japanese, too. I'd prefer それだけ ("that's it") if I want to tell the other person that I'm only asking for one thing.