[elf25]

Bifurcation occurs at the Great Lakes, where the Mississippi flows into the gulf and the St. Lawrence empties into the Atlantic. This forms the “great loop”, a circumnavigation course known by many power boaters . This makes about half of the United States and island by your definitions.

[bryanlarsen]

The Chicago - Great Lakes connection is man made, so doesn't count per the methodology of the OP.

[Kon-Peki]

The permanently navigable connection is man made.

It was seasonally connected (and navigable by canoe), requiring a very short portage the rest of the year.

[Nevermark]

So North and South America are not the North and South Panama Canal Islands. Got it!