The cans I've seen are yellow cyllindrical buoys and they do have the letter printed on them in black. Here is one guy's list of their coordinates...
Lake Erie Latitudes and Longitudes

I only started trolling for walleye last year. I've caught several, but only limited out once. Mostly, I've found several ways to fail. Avoid these...
- Fish can't see baits through all the mud coming in from the Maumee
- Full moon so they gorge all night and aren't hungry by daytime
- Mayflies hatch for 3 weeks and walleyes gorge on those instead of lures
- Storms break sea grass up into floating islands that foul trolling lines
- Water temp too high at islands and points west, so have to go east where it's deeper
- Marking walleye, but marking no bait fish there, so those walleye aren't hungry
- Using Bandits when they want Smithwicks or PWCs