The verses say a barrier which can not be transgressed, so if water is mixing the verse is wrong and so was Allah. A barrier should prevent mixing especially if it can not be transgressed. We see the opposite
Negative you are way off. Water A has less salinity but is cooler than Water M. Water A flows into Body M from a upper layer as salinity is a greater modifier a low levels than temperature. Water M cools Water A faster than salt diffusion to from M to A. Water A flows a distances into Body M due to this. Where Water A meets Water M, Water A mixes becoming Water M becoming Water M. There is no water B as both have close salinity levels.
Water M flows into Body A at a lower level due to salinity and cools fast than diffusion of salt. This allows Body A to flow a distance into Body M as difusion is slower than the movement of the current of M, as it was with A. However diffusion still happen. Water M becomes Water A. Likewise Water A becomes Water B.
Current for Water A is at a higher depth of M. So the current is always move Water be it A or B in a eastward direction. Water M's current is at a lower depth, so the current always move water westward be it A or M. So neither Water can maintain its unique salinity indefinitely due to factors which generate mixing; wind, tide, currents, gravity, etc. Each Body of Water maintains it's unique properties beyond the point of mixing as the flow of the other Water type is not greater than the total sum of the current Water type. This is due to the limited speeds in which mixing happens due to GRE and speed of light restrictions.
Each Body of water is not homogenous but has a number of layers. One of these layers is actually called the mixing layer. These layers divides water with higher salinity from water with lower salinity. These layers are also contain temperature factors. Warm water is on top as it maintains a higher temperature due contact from sources of heat. Cool water lacks this contact so is at a lower depth. In polar regions most of the layers of water disappear as temperature can not be maintained by sources of heat. Salinity become more homogenous but not total homogenous
The barrier as you call it is in fact the point of contact in to water mixes. So the word barrier is not the proper word to use here. If a barrier can not be transgressed then there is no mixing at all. Water A can not become Water M and vice-versa. You are redefining words to fit your interpretation to match science. This is post hoc rationalization. The fact remains the verse use certain words with certain meaning which makes this verse incorrect.
Also when I looked up the word which you claim is separate and/or mix it is in fact a lie. In the Quran lexicon, a lexicon just for the Quran, it has no such meaning. In fact in a use outside the Quran it still does not have this meaning.
[bar-ee-er]
noun 1. anything built or serving to bar passage, as a railing, fence, or the like: People may pass through the barrier only when their train is announced.
2. any natural bar or obstacle: a mountain barrier.
3. anything that restrains or obstructs progress, access, etc.: a trade barrier.
4. a limit or boundary of any kind: the barriers of caste.
5. Physical Geography. an antarctic ice shelf or ice front.
6.
barrier beach.
7. barriers, History/Historical. the palisade or railing surrounding the ground where tourneys and jousts were carried on.
8. Archaic. a fortress or stockade.
[trans-gres, tranz-]
verb (used without object) 1. to violate a law, command, moral code, etc.; offend; sin.
verb (used with object) 2. to pass over or go beyond (a limit, boundary, etc.): to transgress bounds of prudence.
3. to go beyond the limits imposed by (a law, command, etc.); violate; infringe: to transgress the will of God.
