Campo means public square or space, so this is the courtyard right off the canal facing the church Frari. 

Sargent's early watercolors can be significantly different in tone to his later work. Generally you see  careful brushwork and tight, small, and much more controlled washes of color than his later work. He often, but not always, used this medium in preparatory sketches to help him solidify his ideas on what he wanted to do in his oils. In this case, I'm not sure if a final oil was done of Frari or not. 

This painting was shown at the Paris Salon in May 1881, making it one of the first watercolors Sargent ever displayed in public.[1]

As you look at the painting, we are sitting in a gondola facing the courtyard. The church can be seen, in part only, to the right. The signature circle window and arched side doorway being the most significant. The photo (left) shows the front and side elevation with the campanile (bell tower) in the background. In the photo you can see the door towards the back.

In Sargent's painting, you don't quite sense the scale of the church since he cuts it off at the top, but notice the size of the people in the larger image of the photo.

Inside the church Sargent would make sketches:

Study of Donatello's "Saint John," Friari, Venice 
Date?

Monument to Benedetto Brugnolo da Legnago, Church of Santa Maria Gloríosa dei Frari, Venice

Philip Resheph helped me locate Frari being west of the Grand Canal and just slightly south in lattitude of the Rialto bridge at the bottom of (E4), top of (E5).

 

Notes:
Special thanks to Andy Holzopfel, of Baltimore, Maryland, a friend of the JSS Gallery, for sending me the NYU Visitor's Guide of the Sargent exhibit from which this image comes.

Exhibitions

1) 
The New York University Grey Gallery, August 31 – October 30, 1999 called "John Singer Sargent, Draughtsman: Works from the Corcoran Gallery of Art". 

• See The Corcoran Gallery of Art
• See Overview of Venice
• See the years in review  1880