English of Core Text: Rabbi Yose says: All seas are invalid for zavim and metzoraim and for sanctifying the water of the chatas with them. What is the reason? Because Scripture called them a mikveh, as it is written “and to the gathering of the waters He called seas.”
On the daf / in context: The Mishnah in Parah rules that sea water cannot serve as a mikveh for certain purification purposes. The reason is that the Torah itself labeled the gathered waters “seas,” implying they are not the ordinary mikveh of the laws of tumah.
Type of Learning: Custom (scriptural designation of seas vs. mikveh)What Is Learned: Because the Torah calls the gathered waters “seas,” they are disqualified from certain mikveh uses.