The Rio Grande acts as a natural border between Mexico and the United States.
Mainly the Rio River.
The 49th Parallel
None. There is only the man-made border that separates the US from Mexico.
Mexico and Panama
No body of water separates the US. from Mexico
That would be the Rio Grande river.
Lake Erie
The Rio Grande
Mainland Mexico, as well as the southwestern US, specifically southern California, Arizona, New Mexico and southern Texas.
I'm guessing you want the Gulf of Mexico, since that separates the US from Mexico (to some extent, though the Rio Grande does that more thoroughly), Puerto Rico, and other Spanish-speaking islands in the Caribbean. ><><><><><>< Not Mexico, and Puerto Rico is not a country (it is a Commonwealth of the US) But the Gulf does separate the US from Cuba.
The Rio Grande (not to be confused with a river of the same name that separates the US and Mexico).
No. The Gulf of California is a body of water on the west of Mexico which separates the Baja California Peninsula (south of the US state of California) from the mainland Mexico. The Gulf of Mexico is on the southeastern US and east of Mexico, and US states such as Florida, Louisiana and Texas have shores on that body of water.
The Gulf of California separates the Baja California Peninsula from most of the rest of Mexico. Note that the peninsula is NOT part of the US state of California. It is part of Mexico.