A cool simile for coldness, since jelly by its nature needs to be kept so.  See also a superb maritime simile by Keats to convey the same temperatures.  

Below us the sea sat perfectly still, cold as jelly.

Enjoy our bestellar love letter to this slim classic of travel writing. 

Source: Lawrence Durrell, Reflections on a Marine Venus (London: Faber & Faber, 1960), p.  108

Photo credit: Girl with red hat at unsplash