Shabbos, Chapter 11, Mishnah 4. As we have mentioned previously, the sea and the ocean have the status of a karmelis, and therefore the Mishnah rules: "Hazorek bayam arba amos - patur" - one who throws four amos in the sea is exempt. He is exempt and not liable, because the sea is not considered a public domain.
A shallow pool of water through which the public walks:
However, if we are dealing with a stream or a channel of water that runs through a public domain, and the public walks through that shallow pool of water - "Hazorek l'socho arba amos - chayav" - one who throws four amos into it is liable, since this place is judged to be a public domain. And what is the measure of such a shallow pool? Less than ten tefachim. If the depth of the slope descending into the stream is ten tefachim or more, the place is no longer considered a public domain, but rather a karmelis.
The Mishnah then continues and apparently repeats the very same law: a shallow pool of water through which the public walks, and one who throws four amos into it is liable. Why, then, did the Mishnah teach this law twice?
The Gemara explains that there are two things it comes to teach us:
The width of the pool: A shallow pool of water less than ten tefachim deep - even if it is more than four amos wide, and at times some passersby do not bother to walk through it but go around it instead - since not everyone avoids walking through it, its status is still that of a public domain.
Walking with difficulty: Walking through a shallow pool of water involves a certain amount of difficulty, and it is considered walking "with difficulty." Even so, walking with difficulty is still called walking, and it counts as public passage - therefore the place is judged to be a public domain.
Walking with difficulty versus use with difficulty:
This principle was stated only with regard to walking. But a use that is carried out with difficulty, and not by way of walking, is not considered a use at all. The Gemara sets two cases opposite one another: a pillar in the public domain and a hole in the public domain.
A pillar in the public domain: It is nine tefachim high and four tefachim wide, which are the measurements of a karmelis. However, since it is convenient for the public to make use of it and they readily place objects on it, this is a use that is not carried out with difficulty, and the pillar is considered part of the functioning of the public domain - even though its dimensions are those of a karmelis.
A hole in the public domain: It is nine tefachim deep and four tefachim wide, which are the measurements of a karmelis. Although one might have argued that the public uses it to store objects inside it and the like, such use is carried out with difficulty. A use that is not by way of walking and is carried out with difficulty is not considered a use and is not considered a function of the public domain, and therefore we judge the hole by its dimensions alone - and its status is that of a karmelis.
In summary: The sea is a karmelis, and one who throws four amos in it is exempt; but a shallow pool of water through which the public walks, whose depth is less than ten tefachim, has the status of a public domain - even if it is more than four amos wide, and even though walking through it is done with difficulty, since walking with difficulty is still called walking. Use with difficulty, on the other hand, is not called use, and for that reason the pillar is considered part of the public domain while the hole is judged by its dimensions as a karmelis.