Explain --
The water like a witch's oil burnt green, blue and white
it just means dat as d witch's oil changes colour in d same way d sea's water was also changing colour from tym to tym. As it was too hot it was boiling also. The colour change was described as green due to d slimy creatures of d sea, blue coz its d natural colour of water, n white due to the foam formed by d bubbling water.
Hope dis helps