Commentary Par XII 49-51

Torraca (DDP Torraca.Par.XII.56-57) thinks that the waves are found on the surface of the Ebro, the river running two miles from Dominic's native city, an argument contested vigorously by Bertoldi (Bert.1913.1), pp. 45-46, who supports the early commentators' belief that the reference is to the Atlantic Ocean.  Others specify the Bay of Biscay.  After Scartazzini (DDP Scartazzini.Par.XII.49-51), however, the ruling understanding is that the passage refers to the Gulf of Gascony, a more precise location off the Atlantic shoreline.

The sun hides itself from human sight when, at or near the summer solstice, it sets beyond the sight of those on land, because it has moved so far out over the Atlantic.  For Dante, we must remember, to the west of the Gates of Hercules lies 'the world where no one lives' ([Inf XXVI 117]).