I had previously recommended OpenCPN to another poster, my recommendation still stands, just make sure you go to the right site
It will do everything you want, including tides, currents, lots of cool plugins etc.
The difficult part is charts, the only place you can officially
get Canadian charts is from the Canadian Hydrographic lot, and they will relieve you of about $400 for the privilege of giving you the charts that your tax dollars already paid for. You can get NOAA charts for free, but they aren't good for Canadian waters. You can also sometimes get hold of other charts out there on that internet thing if you know what I mean.
As for GPS itself, if the Garmin you have will output NMEA 0183 then you might be able to use that via a serial port (or a USB), but it might prove easier to just get a GPS dongle. I would go for something wired as otherwise you can guarantee that the dongle will run out of power just when you need it most.