I'm not Canadian - I'm a NZer - and our booking is in A$ (gof figure) - my understanding is that the HIA included the 18% gratuities and was in A$ - as is the cruise fare and the pre-paid gratuities. We didn't bother with the HIA as 21 ports out of 25 days we'll won't be drinking that much on ship.
The exchange rate may well be in your favour - I plan on pre-paying the gratuities as paying it in A$ is a quite a bit cheaper than paying the US$ equivalent thanks to the exchange rate HAL is using - worth checking for C$. Unfortunately, none of the other pre-pay options are in A$ - just the gratuities and HIA - very odd.