Yes, there is a line. There are confluences you can see and touch and they are NOT subtle in the slightest.
That's the Atlantic and the Caribbean on a particularly pronounced day.
This is the Indian and the Pacific. It's not always this obvious everywhere but the dividing lines are very much there.
Oceans have their own properties as far as temperature and salinity and unless something like a storm or a current forces them to mix they won't. Mostly this applies to vertical mixing and it gives you things like thermoclines and haloclines but water is wierd and won't mix horizontally either.
The ocean basins tend to have their own currents that go in a circle and define that ocean, and those patterns mix the water within that ocean. Like a washing machine.
The Caribbean has a little loop of its own that not on this map, but that current keeps that ocean pretty internally consistent. It's got clear warm water because of the shallow bowl of limestone sand it sits in. Where it meets the Atlantic with wildly different conditions the water is traveling in opposite directions, and it acts kind of like an oncoming lane of highway traffic. Species that have adapted to a narrow band of temperatures and salinities (most fish) can't cross, while species with a stronger homeostasis hang out there on purpose, (marine mammals, turtles, sharks). Plankton, that cannot control their horizontal movement in the water column, are held in their home territories by these barriers.
