Which ocean borders the mainland USA to the east?

That would be the Atlantic Ocean. It is directly in the east of the U. S.

Down in the southeast, there is the Caribbean Sea but its not an Ocean.

Northeast, theres the Arctic Ocean but its not bordering the U. S.

These are the large bodies of water touching the eastern U. S

Which means that the Atlantic Ocean is the only Ocean touching the Eastern U. S

The Atlantic Ocean would be the answer