Why do you think the narrator mentions "impaired sight" and "befouled bodies"?
Piggy said nothing but nodded, solemnly. They continued to sit, gazing with impaired sight at the chief's seat and the glittering lagoon. The green light and the glossy patches of sunshine played over their befouled bodies.
There's the passage you're thinking of. It comes just after Simon's death, and so the odd description - and the ominousness of it - reflects their guilt about what has just happened. Piggy's sight is impaired, of course - but as for the other two, who don't have eyesight problems, their inablity to see clearly (particularly as they're lookng at the chief's seat - the symbol of control and democracy) reflects the way the civilisation of the island has descended into savagery.
Their bodies being "befouled" - dirtied - has a double sense, I think: of course, after so long on the island (and after the "pig hunt" which kills Simon) they would be actually dirty and muddy. But also, their "sin" - killing Simon - means that they are in some sense spiritually befouled too: spotted with sin.
Another instance of the way Golding often subtly inserts information about how the characters are feeling into the landscape of the island.