You can buy your Canadian Fishing License right on the ferry in Marine City. Just stop in the Customs office first and show them your ID and let them know what you are going to do. Your license will be made out right there and there is no surcharges or added fees. It's just that simple.
If you live anywhere near Marine City you can get on the Ferry and get your license without getting off the ferry they sell them right on it. When you get to customs you just let them know that you want to get license from the ferry and they give you permission. What ever you do, do not walk up behind a customs agent if he happens to be outside talking to someone in the customs office.
The license is sold on the ferry so don't get off when you get to the Canadian side. No passport needed. They also have Walpole licenses. The guy who directs the cars will sell you the license.
You can also order the Ontario license online from Wally's Bait & Tackle (which you can link from LSC Muskie Shop. I think orders go out on Thursdays, and you'll get them about the following Tuesday.
Enhanced driver's license is the easiest way to get crossing credentials, which you can apply for at many Sec. of State offices (state website has a list for each county online). There's also the Nexus card, which is good for a couple more years, but has a more involved application process.
I don't think you need these credentials just to fish Canadian waters from a boat if you don't dock or make contact with other boats, if I recall correctly. Dunno about anchoring. Someone correct me if I'm wrong here, but I plan on waiting until my enhanced license comes in anyway.
Could save lots of gas if you want to launch from the Canadian side, as I plan on doing once my enhanced license arrives.
If you want to save money and can wait about a week or less then call the Ontario Ministry of Natural Resources at 1-800-667-1940 They will tell you it can take up to 4 weeks to get it but it usually comes in the mail in 4 to 7 days.