This cover is similar to the covers of Jaws and The Iceberg as it shows a view of both above water and underwater and the colours of each contrast well. Unlike the cover of The Iceberg, the type is not lost in the colour. It also uses letters of different sizes to help it stand and out and fit around the curves of the main image.
We tried loads of photographic approaches for this cover before arriving at this. I created this illustration as a last-minute entry to the Cover Meeting, and amazingly it ended up being the finished jacket. Here's the finished cover, plus a close-up of the illustration.