This was an awesome experience. I would highly recommend adding this to your itinerary if you are visiting Montreal.
The first 10 minutes of the ride were relatively relaxing. We got some nice views of Montreal and our hilarious guide (Todd) gave us some interesting city facts. After that, prepare to get soaked! The rapids/waves can get pretty intense and it felt like a mini-roller coaster ride on water. The first row pretty much got slammed with the most water, especially the individuals sitting on the edges. My group and I sat in one of the middle rows and we got our fair share of the water. We went on a blazing hot summer day, but after the first several waves, I was already shivering. After the fun is done, the wind on the ride back will help dry you off a bit.
TL;DR: here are some quick tips:
1. Bring a towel, wear a swimsuit during the ride, and bring a change of clothes for afterwards (they have bathrooms/changing rooms)
2. There is an area to store your valuables
3. Do not bring your phone or camera unless it is water-proof or highly water-resistant
4. The optional poncho will not protect you from water (you are guaranteed to get wet), but it can be used as a windbreaker
5. Wear their booties unless you brought your own flip-flops or water-shoes
6. If you sit in the front row, prepare to have water crash into your face