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.”
Type of Learning: Scriptural designation of seas vs. mikvehWhat Is Learned: Because the Torah calls the gathered waters “seas,” they are disqualified from certain mikveh uses.