I've always pictured the boats moving right, sliding down, as if surfing the wave.

The mirrored version makes it clear that, no, they're going against the wave, which makes it that much more of a scary situation!

Now, having noticed that, I see how the position of the rowers in the boat would be enough to deduce that. But still, it goes to show that (at least for me, personally, in this specific case) the mirroring really did bring a more intuitive feel for what the artist was trying to represent.

When the mangaka is creating the layout, they're conceptualizing the flow not only of the panels, but also of the text inside panels, to be RTL.

Translating the text into a LTR language without mirroring the image, makes it so that your eyes have to zig zag around a bit more, going RTL panel wise, but LTR text box wise.

Compared to the problems that mirroring the art brings, I still think that's best compromise of the options, but doesn't mean it's not an actual impact on the experience, even if a subtle one.

I have wondered before, though, about how had might it be to learn to read mirrored, RLT english. Might be a bit of a challenge at first, but would enable you to read translated manga RTL with no compromises (other than the inherent lossiness of language translation in general).

However, when it comes to the actual text (regardless of the medium), it is always written either top to bottom or left to right. There is no right to left text writing in japanese. This isn't arabic, where text is indeed written right to left.

You can see horizontal train stop signs written right to left in “In This Corner of the World” anime. Today all signage seems to be left to right.

[edit] The history section in Wikipedia explains that this was a postwar script reform. https://en.wikipedia.org/wiki/Japanese_writing_system

That said, as I implied in my other reply, the whole idea is a bit silly...