Like Michel said, a mini bus is a good idea. Notre Dame is always congested between Place d'armes and st laurent. They've dug a whole about 4 weeks ago and to this day in the mid section of the street , you can only use one lane due to the construction signs and blockage.
Many times I've seen cars parked illegally in front of the bus stop during business hours.

Definitely making the route information available at Tourist pamphlets will help to get noticed.
And making De La Commune, pedestrian only, access to horse rides and buses is a great idea also.

– Use a minibus, similar in size to those used to carry disabled people and on the 251 in Sainte-Anne-de-Bellevue. A minibus is more flexible in traffic and spends less fuel.

– Using a smaller vehicule would allow the STM to increase the frequency. Make it something that is easy to remember. Every 15 minutes, with service on :00, :15, :30 and :45 at the major stops. Service every 22-25 minutes doesn't make the schedule easy to remember and the last thing a tourist want is to look at a paper schedule.

– Make more streets off limit to car drivers and have the 515 use them. (I see you already cover that part.)

– Finally, why not have downtown hotels and businesses give free day passes valid on the 515 only to all their patrons? It could be a way to get more tourists in the bus and less in cars.

It is too confusing for people to take this route, no one really knows where it goes, or what direction they are going in.

People won't wait for a bus that they aren't sure is going to come, most tourists have no idea what this route is all about unless someone tells them.

We need flags that say something like "Vieux Port Express" or show some sort of recognizable logo so people can instantly know what it is, the number 515 means nothing to anyone, not even daily riders.

Intervals are quite silly with this bus, and ridership isn't helped by the confusing route layout, it has the potential to serve certain areas that have a lack of transit, but it's just too hard to use. The stop location at Berri Metro is only good because it dosen't obstruct traffic, across the street, going the other way might be better, but that would also confuse the routing even more.

All in all, this route is a waste of buses, we shouldn't be running full length standard buses on it, and the fact that this is the pre-amble to our first tramway line, is even more troubling. Why are we putting tourists before the citizens, we live here, not them.

If you have a bus line aimed at tourists, they will gladly ride it if they can see outside of the bus. Even though I am not a tourist, I enjoy watching outside of the bus, and I particularly hate those buses that are not much different from riding inside a truck box.