Lake Superior is about 350 miles east to west by about 160 miles north
to south but you think a sub hiding in there would be easy to
target with megaton ICBMs dropped on the lake? Well I would argue you
would have to drop so many on the lake and stagger them so much
that the sub would be able to return fire even if it does not launch
on warning or under attack. Its average depth is about 500 feet with a
max around 1400 feet.
I don't know if the explosive shock of a explosion is more effective
in water or air. I would think the pressure wave would pack more punch
in water but be
less effective overall as the range increases b/c of the water
absorbing the blast...IE. moving all that water takes energy. An
easier solution may be to target the falls
with large yield weapons to carve the falls out much more by way of a
nuclear fireball. Surely dumping a sub over the falls would kill it
but maybe not before it could
still launch.

JK

I know they're out of the inventory, but nuclear depth charges were
pretty much the more probable usage of nuclear weapons. Great sub
killers. Put the sub in a lake its pretty much dead. Not to mention with
you'd have to build it in place. Add on all the maintenance and other
facilities, yeah, its a dead duck. Much easier than there's a boomer
in the Barents or under the ice cap.

Tell a targeting geek there's a sub in these lakes, much easier than
there are X number of subs dispersed off the coasts. You could even
find those nasty types that like putting nukes in place just drooling
to put them in the lake to be detonated as needed. Not that we ever
did stuff like that ... yeah right.