Very interesting!
An estuary is a place where freshwater from a river or stream flows into a partially enclosed seawater environment. There are several ways to create an estuary, and that really determines how the salt water and fresh water are mixed together. It seems like you are thinking about the fact that freshwater will often flow over a saltwater body and not readily mix together because the fresh water is less salty and less dense. But ultimately, these water bodies do mix together. Sometimes that mixing can change within a single day because high tides can increase the amount of dense, salty water flow into the fresher water. This means that the water at the surface can still taste salty, because even though the surface water might be from a freshwater source, it is still mixing with the salt water. If you could take a taste test from several places within the estuary, it is likely that all but the freshest incoming water would taste salty. But within the salty water, there would be different intensities of saltiness. We refer to this slightly salty, mixed water as "brackish."

Salt water is more dense than fresh water, so fresh water will float on top. However, salt water is just fresh water with salt dissolved in it, so that means the salt in the bottom layer of salt water can dissolve (diffuse is the actual word to use here) into the fresh water- turning it salty. However, the bottom layer will still likely stay more salty than the top layer of water. You might think the water on top tastes salty, but the water on the bottom would taste much saltier.

In an estuary, the fresh water tends to float over the saltwater, but the salt starts to spread into the fresh water and makes it saltier than when it first enters the estuary. Also, tides, water currents, swimming animals, and boats can mix up the water. If you tasted the water at the top and bottom of an estuary, you would probably find that they are both salty, but the bottom is saltier.

Different types of estuaries have different degrees of mixing between freshwater and saltwater. The type of estuary where fresh water flows over salt water is called a salt wedge. In a salt wedge, seawater moves inland along the bottom and freshwater moves seaward at the top. As the estuary goes out to sea, the top layer of freshwater becomes thinner, so we would taste the salt from the seawater at shallower depth. If we travel the other direction, we would find that the freshwater layer goes deeper and deeper, so we would not taste salt until we go to larger and larger depths.

There are many non-salt-wedge estuaries, and the water in some of these estuaries -- vertically mixed ones, for example -- would taste salty. For some more information on types of estuaries, please go to this site.

The salt water mixes with fresh water and becomes brackish water. Brackish water is less salty than sea water, but is saltier than fresh water.

Yes, fresh water does float on top of salt water for a short time, but eventually they mix and become brackish.

Water will mix, so there won't be any totally fresh water. The water on top will just be less salty than the heavier saltier water below.