Mikvaos 5:4

All seas are like a mikvah, as per Genesis 1:10, “The gathering (mikvah) of the waters He called seas.” This is the opinion of Rabbi Meir. Rabbi Yehuda says that only the ocean is like a mikvah; it only says "seas" because there are many kinds of seawater in it. Rabbi Yosi says all seas purify when flowing but they are unsuitable for a zav, a metzora and for preparing the purification water.

Mikvaos 5:5

Flowing water is like a spring and dripping water is like a mikvah. Rabbi Tzadok testified that if flowing water exceeds dripping water, it’s valid for immersion. If dripping water was made to flow, one may position even a rod, a reed, a zav or a zavah (to block the flow), then go down and immerse; this is the opinion of Rabbi Yehuda. Rabbi Yosi says that things that are susceptible to ritual impurity can’t be used to block the flow.