Take this free ferry twice: once to see the river by day and again to see the downtown New Orleans skyline by night. One of my all-time favorite travel photos was shot from the upper deck of the Algiers ferry.
The boats leave from the foot of Canal St. Frequencies vary according to time of day and river conditions, but, with the possible exception of Sundays, there should be at least one trip every half-hour.
The community of Algiers has some historic buildings, making a stopover worthwhile. Two or three blocks up-river from the ferry terminal, there’s a factory that assembles many of the floats for the Mardi Gras parades. If it’s a working day, see if they’ll invite you to go in and walk around. (I was invited in when I was there.) This is living New Orleans history.
If you’re just looking to see the river, it’s o.k. to stay on the boat at Algiers and go right back. However, for another view of the river and skyline, from a very high bridge, leave the ferry at Algiers and take a bus back downtown.
There’s another free ferry, from the foot of Jackson St. to Gretna, but there isn’t much to see on either side of the river.