What body of water did the Roman republic border in 500 bce?

Mediterranean

The great mediterranean became a Roman Lake. these lands would remain under the control of Rome throughout the Republic and well into the formative years of the Roman Empire

In 500 BC, the Roman Republic bordered the Mediterranean.

Explanation:

The traditional date of birth of the Republic is during 509 BC, after the demolition and expulsion of the monarch from Rome and the year of consecration of the Capitoline Triad Temple. After the Etruscan power weakened, the first centuries of the Republic saw the progressive conquest of mainland Italy by Rome. The instrument of the conquest, the legion, was composed of citizens, recruited in wartime. As it advanced in its conquest, Rome used the contingents of the dominated and allied cities as auxiliary troops.

Rome defeated the peoples of Lazio, the Etruscans, the Gauls, who had settled in the Po plain, the Samnites, and the Greek cities of southern Italy, who, despite the intervention of the king of Epirus, Pyrrhus, was conquered by Rome between 280 and 275 BC.

From the middle of the third century BC onwards, Rome, which already dominated the entire peninsula of Italy, launched a series of wars that led it to dominate the Mediterranean world.