It looks like they only go downtown on weekends, so it should work for a Sunday trip. From what I'm seeing you would have to get off the bus at 5th avenue and 13th street. I'm going to link the map I'm looking at. It's a different route for the weekends and is easier to read than the weekday maps. Look at the orange route - West 34th Street/Greenwich Village. It looks like you would board at the terminal and then it will go down 34th street and make a right onto 5th Avenue. It travels down 5th and you can get off right on 13th street - just let the driver know. On Sunday it runs between 8:10am and 10:50pm. Call to confirm if possible, sometimes things change based on ridership.
The rest of that loop is it continues on, makes a left onto W. 8th Street, left onto 3rd Avenue. Then takes 34th street back to the terminal.
https://nywaterway.com/UserFiles/Fil...Map_8.5x11.pdf