A floating ‘hover ship’ was recently photographed off the coast of Falmouth, Cornwall and it has been referenced as a fantastic example of a ‘superior mirage‘. According to BBC meteorologist David Braine:

 

The “superior mirage” occurred because of special atmospheric conditions that bend light. The illusion is common in the Arctic, but can appear “very rarely” in the UK during winter. Superior mirages occur because of the weather condition known as a temperature inversion, where cold air lies close to the sea with warmer air above it.
 
Since cold air is denser than warm air, it bends light towards the eyes of someone standing on the ground or on the coast, changing how a distant object appears. Superior mirages can produce a few different types of images – here a distant ship appears to float high above its actual position, but sometimes an object below the horizon can become visible. [source]

 

If you’re interested in learning more about the science behind the mirage, check out this Wikipedia article.