What is this propeller found underwater in the English Channel?
My best guess is that propeller is related to the 19 June 1954 – Swissair Convair CV-240 flight which was ditched in the English Channel near Folkestone due to fuel exhaustion. As @AEhere mentioned earlier it looks like the propeller wasn't under power.
the propeller shape look similar but still the size of the propeller being provided in the photo relatively small compared to the actual aircraft propeller.

Sources : 

Wikipedia:https://en.wikipedia.org/wiki/List_of_accidents_and_incidents_involving_airliners_in_the_United_Kingdom

http://www.sr692.com/fleet/15_cv240/index.html