Seawater immersion

The Gemara in Tractate Makot (4) says regarding a barrel full of water that fell into the Great Sea, that one who bathed there did not incur a baptism. Some of the early scholars consider a barrel full of “wine” because there is a law of immersion in water, as opposed to wine.
But the Ritva cites the Rav, who cites Rabbi Moshe the preacher, that a barrel of fresh water that fell into the Great Sea, whose waters are salty, is not considered water because they do not mix with each other. Therefore, he rules out a barrel full of “water.”
His words seem puzzling, since the law of irrigation is like ‘sowing,’ meaning that the reason is not because they interfere with each other, but rather that they become connected, but everything remains in its place, like sowing.
(Even his argument that they should mix with each other is puzzling, as ultimately the molecular structure of seawater is the same as freshwater, except that it is a solution of H2O, sodium, etc., meaning that in terms of the mixture it is the same thing).
Is there a way to reconcile their words? Ultimately, it is a halachic issue whether or not there was a baptism, with the strict interpretation being based on an error in both reasoning and facts.