On that cruise, the only Canadian port is probably Vancouver, so tip in USD. The crew only have one day a week in Vancouver, and it's a turnaround day, so there's not a lot of time to go out and spend (or exchange) it. I'm Canadian, and I have zero confidence in the USD right now, and it's a bummer to have to change it, when you know you're losing on both sides of the deal, but it's gotta happen. They can use USD at home (for the time being), but they probably won't be able to use CAN for a while.
I'm expecting this will change when the USD implodes under the bailout money and lack of direction in the economy, and the world reserve currency will shift to either the Chinese RNB, or something the IMF/World Bank cooks up (like an SDR), but for now, the USD is still the king of fiat/debt based money.
It is a good time to be Canadian though...gotta love having a stronger dollar than our little brother down south again. I think it might stay this way for a while now, until the USD collapses, it's game over, and the ships move off to Asia and Europe, where the money is...leaving the Americans in their FEMA camps, with Jersey Shore on, convinced that they still live in the best country in the world.
Use your credit card for most everything, and only exchange enough for tips.