Question

The Talmud in tractate Makkot (4a) says regarding a barrel full of water that fell into the Great Sea, that one who immerses there does not have a valid immersion. And some of the medieval authorities (Rishonim) have the reading "a barrel full of wine," since with water there is a rule of contact-joining, unlike wine. 
But the Ritva brings the Raavad, who cites Rabbi Moshe the Preacher, that if a barrel of fresh water fell into the Great Sea, whose water is salty, there is no contact-joining here because they do not mix with one another. Therefore he reads "a barrel full of water."
At first glance his words are puzzling, since the rule of contact-joining is like "planting," meaning that the reason is not because they mix with one another, but because they become connected, while each thing remains in its place, like planting. 
(And even according to his reasoning, that they must mix with one another, his words are puzzling, because in the end the molecular structure of seawater is identical to fresh water, except that it is a solution of H2O, sodium, etc.; that is, in terms of the mixture it is the same thing.) 
Is there a way to reconcile their words? In the end this has a practical halakhic implication as to whether the immersion counts there or not, when the stringent reasoning seems to rest on an error both in logic and in the facts.