Hool, I would think twice about using US$ travellers cheques, unless you like the 'safety'. In addition to some of the hassle mentioned by disneyholic family, you will pay more in service charges and exchange rates than you have to. If you buy with Visa on the islands, the exchange should be the local currency to the Cdn $, and generally the loonie does better against other currencies than the USD.
It's easy -- and may be cheaper -- to withdraw the US cash you think you'll need at any ATM in the US. And on board, everything, including tips, can be charged to the KTTW card, which in turn is charged against your credit card.