A different non-human race appears, and while the topic of their relation with humans doesn't exactly get much attention, there are a few interesting elements. The painting in the prefects' bathroom compared to the actual Merpeople, indicating the propensity of magical humans to believe in stereotypes and their ignorance about what the non-humans really are like. Somewhat analogous to the way Western society used to view foreign and exotic cultures, liking to see them depicted in art or books, but in rather exaggerated and sensationalistic ways. And then their reaction when Harry draws his wand against them - they are clearly afraid of wizards/witches and their wands, even if they don't know very well what they can do with them. Bit of a faux pas by Harry there in terms of human-Merpeople relations, but then he doesn't think he has a choice (and they don't seem to resent him for it). And then their religion with the huge statue, that was a nice touch - and the domesticated animal.

I wonder about the second task itself. Ron and Hermione are right that Dumbledore wouldn't have let them drown, of course, but how is it that some champions - not just Harry, but Fleur too, evidently - take it seriously all the same. And even Percy, who's actually in the jury, is rather worried about Ron. Would things really have been that simple if one of the hostages hadn't been freed?