These two descriptive paragraphs serve lots of interesting purposes. Firstly, they serve to add to the establishment of the island as a tropical paradise:
The glittering sea rose up, moved apart in planes of blatant impossibility; the coral reef and the few, stunted palms that clung to the more elevated parts would float up into the sky...
There is always something - I think - slightly ominous about the richness and ripeness of the description. And (and here is the other key point) the natural island itself is starting, even at this early stage of the novel, to take on negative, ominous connotations:
Towards noon... the heat - as though the impending sun's height gave it momentum - became a blow that they ducked, running to the shade and lying there, perhaps even sleeping.
The key theme of the novel - that there is a natural evil in people - is also borne out by the suggestion that there is a natural evil in nature. Golding pushes this further:
Strange things happened at midday... land loomed where there was no land and flicked out like a bubble as the children watched. Piggy discounted all this learnedly as a 'mirage'... At midday the illusions merged into the sky and there the sun gazed down like an angry eye.
Mirages - imagined terrors - are seen by the boys. Of course, this foreshadows the imagined "beast", which later provokes the savagery on the island. Natural imagination looms above them in the novel just as the sun does.