In any foreign country count yourself lucky if merchants accept your home currency. Most Canadian retailers will accept US funds but it's beyond any reasonable expectation for them to carry a separate float just in case a tourist wants to pay in US currency. In PEI many of the tourists will be Japanese (Anne of Green Gables fans). Should they keep a supply of yen as well?
I have heard of tourists complaining that when they put their American bank cards at ATM's that US funds didn't pop out. (my bank has a few ATM'S that will dispense US currency but these are few and far between)
We will be camping in Maine in a couple weeks. For most purchases we will use credit cards but we will bring US currency for smaller purchases and places that don't take debit / credit cards.
Having said all that I don't think you'll find a more welcoming and safe destination then Canada. Bring your US dollars and enjoy the slightly favourable exchange rate.