Rome did not try to defend any body of water. She conquered all the lands on the shores of the Mediterranean Sea.
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The Mediterranean Sea.
the nile river
It was the belief of the ancient Egyptians that the way they would experience the afterlife as a soul or "ka" would be influenced by the way their body is prepared through mummification.
I would say the Mediterranean Sea, because you could easily travel anywhere in the known world on it back then, and both Rome and Greece were world powers. It also served as a huge trade route.
Most of the Seven Wonders of the Ancient World were around the Mediterranean Sea.