Is there a good ferry between Kotor and Dubrovnik?

There are no ferries in between the two. You can do an organized day tour, go by public bus (timing schedules between the two country systems may be a pain) or rent a car and drive. I did it via a day tour that hit several spots in Montenegro.

Preliminary information can be found on the BusCroatia website. For Balkan buses, especially border-crossing Balkan buses, it is prudent to check for current schedule information when you arrive in your departure city. I noticed that tourist office personnel dealing with bus-travel inquiries tended to pick up the telephone and call the bus station rather than checking for information online. I suspect, too, that schedules are augmented in-season.

I took a bus from Kotor to Dubrovnik in 2015. It was very comfortable. From Kotor you can pick up a small boat to Perast if you want an opportunity to see the bay from the water. Be sure to allow time for Kotor itself. And check the cruise-ship schedules to try to hit Kotor on a day when there's no mega-ship in port. (Even better if there is a monster ship at Dubrovnik that same day!)

We did Kotor as a day trip from Dubrovnik and it was perfect. The owner of our pension arranged the rental for us. Easy as pie.
Kotor gets mega cruise ships now? :(

Yes, it does. My first day there in 2015 was very peaceful. On the second day I left my lodgings (which were just outside the old town), walked parallel to the wall and turned left to head toward the main gate. There in front of me was an apartment building that hadn't been there the day before. It took me several seconds to realize I was looking at a ship.

I did an Atlas bus tour from Dubrovnik that included Kotor, a lunch stop, Cetinje (the former royal capital of Montenegro) and Budvar (costal resort). The tour departed about 7:00am and returned about 7:00pm, was very good and was quite reasonably priced. Atlas office is located just outside the Pile Gate. Kotor is located on an inland lake, accessable only by a chain of other lakes, so a ferry from Dubrovnik isn't really practical.

Frank, what you're describing is the Bay of Kotor. It's quite dramatic, but it isn't a lake.

You are of course correct, it's a bay because it connects to the sea. Perhaps a bit of wishful thinking on my part, that cruise ships couldn't sail to Kotor if it was on a lake. Thankfully I got to experience the town with no cruise ships in sight.

We took the public bus 3 years ago. Perfectly fine, and not expensive. We stayed in Kotor 2 nights, which was more than enough, since it is a small town. The only flaw was that the Museum of Cats was not open.