Hop-On, Hop-Off Boat Tours in Cinque Terre?

Yes but I don't know as I'd use the ferries as your primary mode of transport. They also don't service Corniglia as it doesn't have a proper harbor, and the town itself sits far above the shore so it's only accessible by train or trail. From the train station, you can take a bus up to the town (small fee) or climb the long, switchback stairway if you've good knees.

Here's the current schedule:
http://www.navigazionegolfodeipoeti.it/en/season-traces-valid-from-21042018-to-30092018/

And here are rates:
http://www.navigazionegolfodeipoeti.it/en/rates/

Click on any of the 5 villages on the sidebar as a departure point. It looks to be generally be 25 euro for an adult day pass with stops between Monterosso, Vernazza, Manarola and Riomaggiore; more to/from other locations.

If you want to visit all the towns in a day, you're better off taking the train. The CT Multi-Service pass gives you one day of unlimited train rides plus you can hike the trails if you want. It's 16 euros, less if you're a senior (which they refer to as "elderly" in Italy.)

When we arrived to the area on our RS tour, we rode the ferry directly from Portovenere to Monterosso. It was a beautiful day, and this ferry ride was one of my favorites things we did on the tour. Lots of tourists were using the ferry that day as a Hop-On, Hop-Off option.

As noted in a previous post, the Ferries do make stops in all the towns except Corniglia, but operation is subject to weather. They run back & forth at regular intervals, but that would be a relatively slow method of transport. You could (for example) take the boat from Monterosso to Porto Venere in the morning, stop at Riomaggiore on the way back and then use the trains from there to the town you're staying in.

The slowness of this particular mode of transport is why I wouldn't recommend it as the primary method of transport but neglected to specify that. Thanks for doing so, Ken!