The water from the Atlantic and Pacific Oceans does mix to some extent, but there are factors that contribute to the separation of these two bodies of water:
Ocean Currents: The major ocean currents, such as the Gulf Stream in the Atlantic Ocean and the Kuroshio Current in the Pacific Ocean, play a significant role in the separation of waters. These currents are driven by various factors like wind patterns, temperature differences, and Earth's rotation. They can transport water over long distances, creating distinct circulation patterns and maintaining the separation between the two oceans.
Density Differences: Variations in temperature and salinity create differences in water density. The Atlantic and Pacific Oceans have different circulation patterns and water masses with varying densities. This can result in stratification, where water masses of different densities form distinct layers, inhibiting the vertical mixing between the two oceans.
Landmasses: The presence of landmasses, such as the Americas, affects the flow of water between the Atlantic and Pacific Oceans. The landmasses act as barriers, limiting direct exchange and causing water to circulate within their respective ocean basins. The narrow channels connecting the oceans, such as the Panama Canal, provide some level of connection, but they do not facilitate complete mixing.
While there may be some mixing at the boundaries between the Atlantic and Pacific Oceans, the overall circulation patterns, density differences, and the presence of landmasses contribute to the maintenance of their separate identities.