I am interested in purchasing a GPS unit and need one into which I can load marine charts (for Ontario, Canada) for use in our boat, and also road maps for the US, for use when we head south during the winter.
Would appreciate recommendations for such a unit.

I.ve got a Garmin Streetpilot 2610 that I use in my Pickup, my big truck and my motorcycle and it works great. Touch screen and easy to use buttons and it will talk to you. Type in address you want and it will take you there. I bought the 1 GB expansion slot so I have about half the US loaded in.
I'm not sure about marine map's but you can look up Garmin's web site and see if they offer it.
Paid about $1,000 total for unit and 1 GB card
They have many different model's depending on your needs, check out there web site and call there tech line too for advise. Find the unit you want then go Internet shopping for the best price. :)

Most Garmins will load the marine maps. I would suggest a nice Garmin permanent mount for the boat. They usually have the waterproofing needed for the marine environment.

i would recommend the garmin 276-c chartplotter with blue chart for the boat and garmin 2620 with city navigator already installed for the truck. im not biased at all.):h

http://www.garmin.com/products/gpsmap3010c/#

Put 'er in the boat, pull it, put a different card, put it in the truck.