"Tell them what? I'll tell them nothing. They're not people like us, they're just a bunch of sardines."
The fish people in the water below do not like this.
"You heard me," jeers Sean, "Cold-blooded fishes. You haven't got a drop of good red blood in your body."
They don't like that either. They've been surgically altered by the regime of Professor Zaroff, an old Nazi scientist who was employed by the Western powers before he disappeared (it's implicit) and who is now running the underwater city of Atlantis (the Nazis were obsessed with Atlantis). He has forcibly turned an army of his workers into fish, complete with gills and fins and big round eyes, so that they can do the underwater jobs. (They just don't make mad scientists like Zaroff any more.)
"A flatfish from Galway would have more guts in them than that bunch!" Sean continues. Oh yeah, I forgot to say... Sean's Irish, hence his "gift of the gab" (sigh).
The patronizing use of Welsh stereotypes in 'The Green Death' is evidence of the employment of centuries-old imperial condecenscion. However, Welshness alone does not straightforwardly equal idiocy in this story. Rather, it is the conjunction of Welshness with membership of the proletariat which produces characters who don't really have a clue what's going and need everything explained to them.
Clifford Jones and 'Nancy' (note how she doesn't need a surname) are allowed to be efficient and useful only because their Welshness (which entails them using cute provincialisms galore) is offset by their educated, middle-class boffinity and right-onitude. Meanwhile, Jo marvels openly at her own foolishness in caring so much about the death of a "funny little Welshman" (who kept her alive). The difference between these Welsh characters - i.e. between the ones who qualify as people and those who don't - is down to class.
The workers in this story are belittled, peripheral figures. They are profoundly out of touch and their Welshness is but a conduit by which they can be further quaintified. They miss the big picture, even when the hippy scientists try to explain everything to them. They side with Stevens ...
The great underwater strike ballot & blockade scene from 'The Underwater Menace'. Seemed timely to me. This is how you fight power-mad maniacs who are determined to destroy your society while pretending to save it.
Well, maybe not exactly like that. You get the general thrust, I'm sure.