HAL will probably have a shuttle, but it will probably be more expensive than a taxi, as HAL charges per person.
I am sure the taxi driver will take US$ but don't expect to get the exchange rate..........US taxis don't take our Canadian $'s!
Why not get some Canadian $'s either from your bank or from a bank at your first Canadian port. Also, check with the front desk when you are on board and see if they have Canadian $s to exchange for you - I doubt it but you can try.
Check this out - although this is from the airport to the cruise terminal in Iberville, the price should be the same.
Taxi and limousine service
No reservations are required for departures from Montréal-Trudeau. Visa, MasterCard and American Express credit cards are accepted. Some drivers will accept U.S. currency, but provincial regulations state that the client must pay in Canadian dollars. We recommend that you agree with the driver on a fixed price for the trip before taking a taxi or limousine.