This premise is incorrect; the oceans do mix. Sometimes on the surface of the ocean, a clear line of demarcation can be observed as two bodies of water interact. This kind of "tide line" happens when there is a density difference due to different salinity or temperature or both. Rather than mixing smoothly, one water body tends to ride over the other - less dense on top, more dense on bottom. But eventually the oceans mix. The worldwide flow of ocean water is called the thermohaline circulation, and as the name suggests it is driven by temperature and salinity differences.