Much have I travelled in the realms of gold
And many goodly states and kingdoms seen;
Round many western islands have I been
Which bards in fealty to Apollo hold.
Oft of one wide expanse had I been told
That deep-browed Homer ruled as his demesne;
Yet did I never breathe its pure serene
Till I heard Chapman speak out loud and bold:
Then felt I like some watcher of the skies
When a new planet swims into his ken;
Or like stout Cortez when with eagle eyes
He stared at the Pacific—and all his men
Looked at each other with a wild surmise—
Silent, upon a peak in Darien.
‘Pacific’ would have suggested to Keats' contemporaries an awe-inspiring ocean, continuing the thread of the exotic and far-flung mystery. 
 
 The dash represents the idea that Cortez is distracted by the immense ocean, instead of focusing on the purpose of the expedition and the needs of his men.