Our group has really enjoyed staying in the Melusina Young apartment. It is very spatious, clean and well-situated. It takes less than 5 minutes to get there by foot from the vaporetto station Ospedale, and you can be at Piazza San Marco, Ponte del Rialto and the Biennale venues in 10-15 minutes walk.
Andrea was a perfect host even we haven't met in person. He is very well organized, responds instantly and you can always reach him if you have a question or need something.
I would definitely recommend anyone to stay in his apartment when visiting Venice, especially is you're a bigger group of friends.