Boomers In Large Deep Lakes?

I have always wondered that if the the U.S. and Russia were really
serious about MAD why were boomers ever deployed to the opean seas?
Think about it. The Great Lakes are very large and deep, some over
1,000 feet deep. There is no reasonable way I can think of to attack a
boomber once out on a patrol in Lake Superior. I mean are the Russians
going to smuggle an attack sub in there? No. Use a ship with smuggled
in sonar gear and torps? Yeah maybe in a dream. Doing this would
probably make boomers far less expensive to design and operate as you
probably would not need all the systems a sea going boomer would need.
You could probably save on missile design by being able to launch from
the surface...

Just wondering what some opinions are.

JK