"from where the Doges of Venice had once set out to wed the seas with rings."

St. Mark's Square is the main square of Venice (marked '5' on the map at the back of the book (Afterword p. 7).

St. Mark's Square, Venice, with Basilica of St Mark and campanile in background
Creative Commons AttributionSt. Mark's Square, Venice, with Basilica of St Mark and campanile in background - Credit: Xiquinho Silva

The Doges, or rulers, of Venice, whose palace (now a museum) overlooks the square, took part in "marriage of the sea" ceremonies to celebrate their maritime domination and pray for protection.

 

Doges' Palace looking onto St Mark's Square
Creative Commons Attribution Share AlikeDoges' Palace looking onto St Mark's Square - Credit: Tango7174