Wanted to give you some input on our impressions, etc of Manzanillo.
The best web site I found was go2manzanillo.com, which gives a good overview of downtown and the beaches, etc.
To answer your questions ...
Yes, you dock in an industrial area. Not so industrial as the Mazatlan port, but more military and other buildings about. It's also gated, from what I recall. They have a free shuttle bus that takes you right into town, and is very convenient for a round-trip return (it shows up very regularly).
In terms of comparing it Tijuana, I would say that is not an accurate description. First, it's much more sleepy town, and very easy to navigate and feel safe on foot. I don't recall any beggars or aggressive sales people. I think this harkens to the fact that it's not a primary stop for the cruiselines. I felt very safe walking about. In terms of dirt, I didn't see any more than was normal. Some Art Deco hotels are run-down looking downtown, but you can still see the architecture that comes shining through.
For most, it's the beaches that are the draw here, but because we are not beach people, Art and I just chose to do a walking tour and take it easy downtown. Made for a very relaxing port stop.
The shuttle will drop you at the marina-adjacent plaza/jardin area, with its large marlin statue (indicating the fishing hertiage). This has got to be the largest plaza areas we have ever seen, and very picturesque due to the fact that it's next to the water. The jardin is lined with white wrought iron fences, has a large splashing fountain and the gazebo, of course. I recall Art chatting with a police officer here, who was very friendly.
We wanted to see the cathedral and its stained glass windows depicting Our Lady of Guadelupe. We just walked from the plaza and headed along the main throughfare that runs adjacent to it up a small hill to the cathedral. Its architecture was unique in that it has open cutouts to let in the sea breezes. It's not as elaborate as a lot of the cathedrals, but the windows are pretty.
We tried to find the cultural museum by asking locals, but were unable to do so. Maybe you will have better luck.
By the way, if you walk in the opposite direction on the main thoroughfare and head toward the hotel area, there is an internet cafe there.
We also strolled farther north past the plaza to the fishing pier area. Yep, there was a smell there, but we could say we did it. I wouldn't go back again though ... not much to see.
Also, on the main drag running parralel to the plaza is the goverment building. Here, an ongoing photo exhibit depicts life in Manzanillo, as well as the destruction wrought by hurricaines, etc. The exhibit was being revamped when we were there, but a government worker sat us down in his office and pulled out book after book of old photos and explained their significane (hurricaines in the '30s and earthquakes). Of course, he spoke in Spanish, so Art translated for me. I was quite impressed how this man went out of his way to show us the photos, etc. Just shows you how friendly Mexican people are in the smaller towns.
After we wandered into town and just took some silly photos and sucked up local color, we stopped at the gorgeous Hotel Colonial. This I would say dates back to the '30s and had beautiful 1930s style Mexican architecture in the interior, with lots of wood used and colorful tiles. The restaurant/bar area was large and breezy. Although we just happened upon the restaurant, there were a lot of people from the cruise ship there, so it's obvious that it comes highly recommended. Service was pleasant, and the food reasonable priced and simple Mexican fare.
At about 3 or so, we were pretty much walked out and ready to head back. We waited about 10 minutes for the shuttle and chatted with other people from the cruise ship. All in all, it was a very relaxing day of exploring.
For others, the beaches are the main attraction. This was, after all, where "10" was filed with Bo Derek. The beaches are listed on the web site. I do know from past posts that one is quite near the cruise docking area.
Hope this all helps, and check out the web site for more details on street names, etc.