The Long Canal is a large canal, and one of the main waterways in Braavos.
In the books
In the A Song of Ice and Fire novels, the Long Canal and it's location are presumably the same.
There are two primary canals in Braavos: the Canal of Heroes and the Long Canal (along with numerous mid-sized ones and many smaller ones). The Canal of Heroes runs from the Titan of Braavos to the central lagoon - where temples such as the House of Black and White are located. The Long Canal, meanwhile, is on the opposite side from the Canal of Heroes, though set off at roughly a right angle. Both the Purple Harbor (where Braavosi ships dock) and Ragman's Harbor (where foreign ships dock) are on the same side of the city as the Titan. Thus in the TV scenes when Arya first arrives in Braavos and Ternesio Terys takes her to the House of Black and White, they actually must have used the Canal of Heroes (given that aerial shots of the entire city match the maps from the books).
Mace Tyrell never travels to Braavos in the timeline of the books, and so his tuneless rendition of the Braavosi shanty is never heard.
· Moon Pool · Palace of Justice · Sealord's Palace · Temple of the Lord of Light (Braavos)
· Temple of the Moonsingers