I understand that #1 and #2 are the main lines, but am uncertain if they are like a subway going in both directions from their stops or more like a hiking trail.
I think that #1 is "out & back" and #2 is unidirectional in a circle - but would like to confirm that detail. This would be helpful to clarify in the next edition of the Venice guidebook.
You would do well to get the CheBateo app which gives great detail of all the vaporetto lines.
Number 1 is the all stops workhorse which starts and finishes at the P.le Roma, where the land buses and taxis come. It goes all the way down the Grand Canal and then after S Marco hugs the the main islands of Venice before hopping across to Lido where it ends (and starts).
Like any city bus it just goes back and forth, about 10 minutes between.
The number 2 is actually comprised of 3 sub-routes, but they are not broken out that way. The main part of the number 2 that most people come into contact with is the semi-fast vaporetto which calls at fewer stops and runs between P.le Roma and S Marco, calling mainly at major stops. Like most buses it goes both ways. The second variant is much the same but it only gets half way down the Grand Canal from P.le Roma and terminates at Rialto. Then it goes back. The third variant starts at S Marco and goes around the bottom of Dorsoduro calling at S Georgio and Zatarre before making its way along the Guidecca Canal (not the Grand Canal) back to P.le Roma. It too goes both ways.
As the number 2 passes through P.le Roma you can stay on if you want to switch from Grand Canal to Guidecca Canal or vice-versa. However when passing through S Marco (Valeresso stop) you need to change because the boat doesn't keep going in a circle there.
Does that make sense? I've never heard of buses or vaporettos being compared to a hiking trail.
One notable difference to a bus:
Buses driver on the different sides of the road depending on which direction they are going. The stops on one side of the road are for one direction, the ones on the other side for the other direction.
Not so for Vaporetto, the same stop is used for both directions. In the Grand canal, the stops are alternatively on opposite sides of the canal. This is useful, as crossing the Grand Canal on foot is not easy. Just look which way the Vaporetto is going.
