There is even a special form called a Form T1135 – Foreign Income Verification Statement that is meant for reporting foreign assets and income to the CRA. It is used to report “specified foreign property” if your cost for certain foreign assets exceeded $100,000 Canadian (based on the exchange rates at the time of purchase) at any time during the year.
Most “investment assets” like U.S. stocks would be considered specified foreign property, whereas personal-use real estate like a vacation property that is not rented out and only used personally would be excluded.
The treaty requires 15% tax withholding on dividends and 10% tax withholding on interest. So if you own a U.S. stock, as a Canadian resident, there will be 15% withholding tax on any dividends earned.