Celebrating England's naval prowess was the original intent of "Rule, Brittania!," the song on which Beethoven based this set of theme and variations.
The opening words of the chorus read, "Rule, Brittania! Brittania rules the waves. Britons never will be slaves!"
The author, a Scotsman named James Thomson, spent most of his life in England, and wanted to foster a unique British identity, rather than the separateness of being English, Irish, Scottish, and Welsh.
Beethoven was a master improviser and a fan of music from diverse cultures. He set songs from the British Isles frequently and apparently found "Rule, Britannia!" so intriguing that he used it twice!
The other place the tune appears is his composition "Wellington's Victory" which commemorated the Duke of Wellington's defeat of Joseph Bonaparte near Vitoria, Spain.