Salt water is denser than fresh water, so it is POSSIBLE for a layer of fresh water flood to float above the salt water of the oceans until the wave actions eventually mixed the two over time.
Of course this is just one of many possibilities.
There are quite a few biological factors along with physics.
The Amazon river of fresh water extends 100 or so miles into the Atlantic ocean.
There are also many accounts of fish species able to endure beyond brackish waters.
One is the bull shark that swims inland into fresh water rivers.
Another example is the transitional lifespan of salmon.
You make an excellent point about buoyancy pollard.
The oceans salinity is not consistent.
What kills fish is the shocking measure scale between acidic and alkaline ph scale.
Like putting salt on a slugg or snail smaller fish cannot handle the degree of change in the waters ph.
Blessings Always