Rif Eruvin 28b {Eruvin 86a continues ... 87a}

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28b

Rabban Shimon ben Gamliel says: Bet Shammai says – Below and Bet Hillel says – Above.

Rabbi Yehudah says: The partition shall not be larger than the wall between them.
{=If there is a wall between the two courtyards which passes over the cistern, there is no need for an additional partition within the cistern.}

Gemara:
Rav Huna said: Below means actually below {below the mouth of the cistern, though it may be above water level}, and above means actually above {near the rim, and there is no need to extend it to the water}, and both of them within the cistern.
And Rav Yehuda said: Below means below the water - that is to say, the poles {of the partition} should dip beneath the water 9 handbreadths and the heads of the poles should be seen above the water 1 handbreadth; and above means above the water - 9 handbreadths and the poles need to dip in the water 1 handbreadth so that this one should not draw water from the domain of that one, and that one should not draw water from the domain of this one.
And our sugya is like Rav Yehuda.

{Eruvin 87a}
Mishna:
If a water channel passes through a courtyard, they may not draw from it on Shabbat, unless they made for it a partition ten handbreadths high at its entrance and at its exit.