Ship convoys, so that you can transport ships across land. Given it would be a slow process but it would makes areas like the Great lakes and Caspian sea more convenient.
The actual St. Lawrence Seaway that connects Lake Ontario and the rest of the Great Lakes to the Atlantic Ocean was not completed until the late 1950s, and did not exist during World War II.
I would oppose adding any form of land-going "convoy" that could transport destroyers and larger ships across land as completely unrealistic. If you're dumb enough to build a battleship in Lake Michigan, you need to learn to live with your mistakes.
The post was edited 2 times, last by MontanaBB ().
@F. Marion: Well, I just learned something: 28 submarines were built in Wisconsin. I can only imagine the surprise of observers watching submarines cruising on the surface of Lake Michigan and the Mississippi River. Of course, many other smaller navy ships and boats were built in yards on the Great Lakes and in the Mississippi River valley. My father was a U.S. Navy civil engineer who was briefly stationed in Pittsburgh, Pennsylvania in 1941, where the navy was building a new yard on the Ohio River to build landing craft.
U.S. Navy submarine "fleet boats" were small enough to exit Lake Michigan via the Chicago River and the Chicago Sanitary and Ship Canal, and then travel to the Gulf of Mexico and the Atlantic Ocean via the Illinois River and the Mississippi River. I just checked the draft, beam and length of Fletcher-class destroyers, and they are only somewhat larger than Gato- and Balao-class submarines, which means presumably they too could have been built in Lake Michigan shipyards (as well as smaller destroyer escort types). That said, none of the Fletchers were actually built in Great Lakes yards.
Cruisers, battleships and aircraft carriers were apparently too large to depart Lake Michigan via the same route. Weirdly, the U.S. Navy did operate a small aircraft carrier (USS Wolverine) on Lake Michigan for carrier pilot flight training during the war, but she was a converted sidewheel steamer that was built and spent her entire career on the Great Lakes before being scrapped. (As an aside, a substantial number of the U.S. Navy's new littoral combat ships are being built in Great Lakes yards.)