I like and use italics as internal dialogue, and have never encountered a good reason why it is supposedly frowned upon.
Telling: The river is long.
Showing: The river stretched as far as POV's eye could see.
Showing or Telling?: Man, that river is long. We're telling that the river is long, but we're showing the reader the character's actual thoughts on the subject.
Third example:
John looked out the window. Why is Alice doing this to me, he thought? She knows damn well how I feel about cheese!"
Because authors who use it do so extensively. It makes sense to format away the italics in the example Benjamin gave us, because it's one sentence (or 3 or 5). But when you get to 10% of your book as internal dialogue (Steven Erikson comes to mind) then formatting it out won't work. You'd have to keep referring to the internal dialogue as "he thought" or "she thought."
My point is, I'm not so attached to italics that I'll argue with and editor about it. It's kind of whatever for me.
Again, I want to preface this with "I really like the practice of using internal dialogue!"
But going by your example,
The river drained into the blue of the horizon. Brian set his jaw as he plunged the paddle into the calm water. The burning in his shoulders protested each push but he had not choice; the mercenaries wanted him dead.
That is showing (as pertaining to the river). Having the MC state the river is long, whether through internal dialogue or actual dialogue is telling. It has been commented before that using dialogue is a trick for telling information to the reader without giving the reader the impression of info dumping. Why would internal dialogue be different?
You have stated my most recent thoughts perfectly with this.This is one of those things.
So the first point is that you have Big Name Authors using italics for thoughts. "Look!" says the newbie author. "[Big Name Author] does it! Why can't I?"
Then the Cranky Editors respond: "Because you're not a Big Name Author. You can't get away with doing that."
This response makes no goddamn sense. It presumes that there exists some objective, universal rule about whether or not you're allowed to use italics for thoughts (which I will henceforth abbreviate IFT), and if you're famous enough, you can get away with it, because who's going to tell Big Name Author what he can and can't do? What they really mean is "We don't like it, so we're going to tell you that you can't do it, because you don't have enough clout to override us."
Even if a certain fraction of readers did have problems with IFT, we must ask whether they really object to it on a visceral level or only because certain writing "authorities" told them not to use IFT. A lot of our ideas of what counts as bad writing have less to do with whether we genuinely dislike the prose and more to do with what these "authorities" say we should consider bad writing. As another example, reading varied dialogue tags (e.g. "muttered", "shouted", etc.) in books never bothered me at all until I read "advice" on the Internet saying never to use any word other than "said".There are no objective, universal rules about writing. There are only guidelines, each of which is more or less compatible with various goals. (Some guidelines correlate so strongly with so many goals that you may as well refer to them as "rules"–much in the same way that thorough, rigorous, time-tested scientific theories eventually become "laws"–but in writing, you can always find a corner case where the guideline doesn't apply.)
Is the guideline "don't use IFT" necessary for achieving your goals? Well, it depends on your goals. If your goal is for non-editors to enjoy your book and tell their friends to buy it, the guideline is irrelevant, because most readers don't mind IFT. Yes, there are some who do, but I'd wager they're a tiny fraction of the population. (Some might argue that IFT are one of those things that people react instinctively negatively to without being able to verbalize, to which I would respond: how exactly could you ever determine that?)
Even if a certain fraction of readers did have problems with IFT, we must ask whether they really object to it on a visceral level or only because certain writing "authorities" told them not to use IFT.
I don't think readers are the ones you have to worry about. Frankly, I doubt most of them care one way or another. If you are going the traditional publishing route, you do have to worry about what editors care about, however. If and editor at a given market is likely to stop reading your manuscript because they think IFT signifies an amateur, it is useful to know that information before submitting.
Let's think of this in the reverse.
Alfred paused before the path leading up the mountain. The ascension was steep. The last sentence is a tell, right? We would all agree upon that and criticize the writer's (mine) use of telling rather than showing. Especially since it's easy to show this information.
Alfred paused before the path leading up the mountain. The ascension is steep. All I did is change the formatting and the tense of the sentence and it became internal dialogue. Everyone wants to tell me that this is not telling?
Yes. That is telling.
As I wrote, you can use internal dialogue to poorly present exposition. However, you imply that all internal dialogue is poorly done exposition. This is not the case.
Let's think of this in the reverse.
Alfred paused before the path leading up the mountain. The ascension was steep. The last sentence is a tell, right? We would all agree upon that and criticize the writer's (mine) use of telling rather than showing. Especially since it's easy to show this information.
Alfred paused before the path leading up the mountain. The ascension is steep. All I did is change the formatting and the tense of the sentence and it became internal dialogue. Everyone wants to tell me that this is not telling?
That isn't what I'm implying. But it is what I've seen most of the time in books. Even the great authors do it. I think that internal dialogue shouldn't be used for outside observations, it should be only for internal....dialogue.
For a character to comment on anything external through internal thought is telling. A character who reflects on the decision he made and how he can reconcile it with his internal moral compass is a rightful use of internal, and thus italic, dialogue.
Should 'That Bastard' be italicised? I think what's going on is clear. 'That Bastard' could be italicised IMHO or not. To me it depends on if you want to emphasise it or not, and I don't think a though tag is necessary.