Roughly two years ago, $1 USD was worth about 95¢ CAD. Right now, $1 USD is worth about $1.26 CAD. That's about a 30% increase in value. The American dollar is doing anything but "plummeting" right now. In fact, the US dollar is trading stronger against the pound than it has in years, and stronger against the Euro than it was a year ago. I follow the market a little bit, and it seems to me that the dollar has been gaining (trade-weighted) value since this recession started.
Another thing to understand is that when the US Government borrows money, it does not borrow from foreign governments, primarily. It primarily borrows from investors, many of which are Americans. The United States' debt is, to some degree, to its own citizens—but as generally only the wealthy can afford to buy bonds, the government generally only borrows from the wealthy.
What this means is that the roughly 10% of our taxes which goes to pay interest on the national debt is going to pad the pockets of already wealthy individuals. Very little of it goes to foreign governments, and the only major influence national debt has on the value of the American dollar is that, if the debt were so high that our government were unable to function because interest cost so much, the dollar would likely become worthless very quickly—potentially almost overnight. As I've shown, however, the dollar is currently strengthening, not devaluing.