[JenB]

I wonder how long I will have to live in the Midwest for the Great Lakes to be an acceptable ocean substitute?

[ExGeeEye]

Depends on you :)

Pros:

See above; also,
Proximity
Visually indistinguishable
Lost in the middle? Rescue more likely
Surrounded by friendly nations

Cons:

No whales, either
Or pirates, arrr
Or ninjas...oh wait...
No seashells washing ashore (shellfish are considered "invasive species")

Visit a few times, see how you like it :)