The Panama Canal.
None. Mexico and Colombia don't border each other, as Central America (Guatemala, Belize, Honduras, El Salvador, Nicaragua, Panama, Costa Rica) lies between both countries.If you mean the most important waterway between both countries (not dividing them), that would be the Panama Canal.
If you mean Middle America as a geographical term (and not the socio-economic term for middle class America) then really its just a pseudonym for Central America so generally speaking, the countries on the isthmus that lies between the contential U.S. and South America. If you're looking for countries then between the U.S. and Colombia. Look here for the wikipedia entry and http://en.wikipedia.org/wiki/Middle_America_(Americas)
Carribean Sea
The Gulf of Mexico lies between the Florida Peninsula and Mexico.
Mexico lies between USA and Guatamala
The pharynx
It lies between the United States and Central America, and between the Gulf of Mexico and the Pacific Ocean.
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Mexico
Mexico lies mostly between 86° and 118° W longitude.
California
Gulf of California