The view in the painting is facing the front of the old Library (designed by Jacopo Sansovino in 1536) the Pizzetta to the right and beyond which leads to St. Mark’s Square from the lagoon where Sargent paints this. You can somewhat see the winged lion of St. Mark in the background (thumbnail left). The freestanding column supporting St Theodor and his crocodile is at the far right of the painting (thumbnail right).

At first I thought we were looking towards St. Mark's square (looking north), but now I believe we are looking east across the front of the Libreria. The lagoon is to our right and the Piazzetta would therefore be beyond the Libreria and heading back left towards San Marco.

From: The Tate Gallery
The artist showed two watercolours of the Piazzetta at the loan exhibition held by Carfax & Co., April 1905: No.31 lent by the artist, and No.37 lent by p. Wilson Steer. Charteris (1927, p.287) lists an oil ‘Piazzetta, Venice’, 21 1/2×27 in., under the year 1905.
(Tate Gallery)

The Library in Venice 

In the watercolor at the National Gallery of Art, Washington, DC, you can see Sargent's interest in a single opening on the second level corner. It's almost a perfect study of Jacopo Sansovino's (1486-1570) use of Ionic order of columns and highlights the interest that Sargent had in architecture.

 

Photo of the Libreria
 
 

Notes: 

• See the Tate Gallery, London
• See the year in review  1904
• See San Marco
• See Overview of Venice