The Atlantic Ocean.
The Northern Atlantic
Lake Erie
That would be the Rio Grande river.
The Bering Sea.
No body of water separates the US. from Mexico
The Bering Strait separates Russia and the US.
bering sea
The Bering Strait
Bering Sea
Canada didn't sell it to the US; Russia did. Canada owned it first, then Russia, now the US. Russia found it useless and hard to get to because of the water body that separated it from Russia.
The Bering Strait splits Alaska from eastern Russia.