You can do this really quickly.
(This is the way I did it, not sure if you DO need the Ferry Depot first.)
The ferry is unlocked at 2,800 population.
Drop down a Ferry Depot ($33,000).
Drop down two Ferry Stops ($5,000 each).
Draw the Ferry Pathway connecting all three.
Place down Ferry Lines between the two stops so it finishes the line.
You don't need to connect services to them
Continue doing it until the achievement pops.