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.
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.
The port is in the fixed price zone (I think it's currently CAD38.00 for a taxi and $49.50 for a limo). When you exit the airport there is a taxi/limo stand. Ask the guy to make sure he agrees that it is in the fixed price zone. It should take about 30 minutes at the legal speed limit.
They will take USD but will give you change in CAD and a horrible exchange rate. CAD38 is about USD31 at the moment, but expect him to tell you USD35 at least. Some will tell you USD38 but that includes the tip. They are quite generous with your tip. (Canadians generally tip $2 for a trip, but maybe a bit extra for the luggage).
If you are going to need CAD in any case, you can go upstairs at the airport where they have a number of ATMs from different banks including the Bank of Montreal, Desjardins Co-op and the RBC (Royal Bank of Canada). Just ask for a "guichet" (geesheeh) and people will point you in the right direction. It will be cheaper than trying to get CAD in the US unless you live in a border town.