I spent 2 1/2 months in Waterloo in 2017, visiting from the USA. In general, I found that my US credit card served me well for almost everything I needed to do: groceries, restaurants, laundromats, day trips to Toronto, etc. I also used my US ATM card to withdraw a supply of cash a few times during the trip, though I found that I didn't need it all that often.
Pace Kate Gregory's answer, I don't recall running into any stores that would only take debit cards, though I don't doubt that they exist. The only hassle involved was the fact that most US credit cards require a signature for many purchases (chip & signature), while most Canadian credit cards do not (chip & PIN). Coming from France, I suspect that this will not be an issue for you.
Before my visit, I sought out and applied for a US card that advertised itself as having no foreign transaction fees to use on the trip. This said, I'm not sure that this really saved me any money, as I didn't actually check into the exchange rate the card offered, and I have a sneaking suspicion that the "foreign transaction fees" were just effectively rolled into the exchange rate I was given.
This said, my rent & utilities during were paid for by my hosts (Perimeter Institute), so I don't know whether credit cards would have been useful there or whether I would have required an Interac card. I also ran into one awkward situation where I paid for a friend's ticket to a hockey game, and she attempted to pay me back with an e-mail transfer (as mentioned by Kate Gregory) that I had no way of accepting. She paid me back with some cash instead, but I remember it being a bit of a hassle for her to "undo" the transfer. This probably had more to do with the level of customer service available at her bank, though.