Justice Robie's opinion is spot on. Indeed, I can summarize this 14-page missive in a single paragraph (of my own creation):
"The watermaster appointed by a court pursuant to a judgment doesn't have standing to appeal a decision of a trial court giving water to a particular person; instead, the other landowners (if any) thereby deprived of water are the only ones who might be able to appeal. Even if the trial court's decision seems wrong, or makes the watermaster's job more difficult, that doesn't matter -- the watermaster is getting paid for its work, so more work simply means more money, and that's not harm. In any event, if the watermaster doesn't like the burden, it can quit. Because it's not harm, it doesn't have standing, so the watermaster's appeal of the decision below is hereby dismissed."
There you have it.
Or, as we sometimes say on the basketball court, "No blood, no foul."