(south) Atlantic
Gulf of Mexico
They flow into 'The Tigris' and 'Ufratis' rivers!
in a river, stream, lake, or ocean
The River Nile flows into the Mediterranean Sea.
The Mediterranean Sea
The Orkney Islands are between the North Sea, The Atlantic Ocean, and the Pentland Firth. Scapa Flow is a large semi-enclosed body of tidal water within the Orkney Islands. It is connected to all three of the above seas.
They flow into the Persian Gulf.
They flow into the Persian Gulf
Rivers
A river is a large natural path water travels in to reach an ocean or other large body of water. A reservoir is usually a man-made lake of sorts. The water does not flow and its purpose is simply to wait until it can be taken for human use.
The Tigris and Euphrates flow into the Persian Gulf
The Mediterranean Sea