About your analysis of the question of water in park:
the halacha you quoted – m”b 44 refers to water which was a later development and so does not retroactively fix the mechitzos [hukaf ulvasof pasach]
if the water was there from the beginning [or intended as such] it would create hekef ldira as i mentioned from rashi – and is brought in the mishna brura 86 ibid. see also the first biur halacha in the siman who brings rashi – and those who argue on him for a separate reason which is not applicable to a city park
so again, what would be the status of a park with a water fountain?
The basic question is what is the definition “hukaf ledirah”. Is it sufficient that the enclosed area is used by people during the day who walk in and out constantly, or or do they have to sleep there as well. From Rash it seems that it is sufficient that it is used by day and so is the opinion of other achronim. Therefore a public park frequented by the public who sit on the benches, lie on the grass etc would be considered hukaf ledirah. But if so how does one explain the Shulchan Oruch 358:1 that gardens and orchards are not considered hukaf ledirah? The obvious answer must be that the Shulchan Oruch is referring to gardens and orchards that are en fenced to protect their fruit not for public use. But still we have a problem why is “dir” used for protecting the animals quoted in the mishnah considered hukaf ledirah more than gardens and orchards. Answers the Biur Halachah (O:CH 358) because the shepherd walks into the dir day and night to attend to the animals. (why doesn’t he quote Rashi Eruvin 19b ד”ה דיר that the shepherd sleeps there at night?) According to this logic if the park is locked at night would not be considered hukaf ledirah. Lemaseh since the park probably has many uses e,g benches, water fountain, play ground facilities,picnic area etc it is probably considered hukaf ledirah.
Rashi Eruvin 22a ד”ה אויר, Teshvas Chasam Sofer O:CH 95, Shulchan Oruch Harav 358:1, ibid Bach,Teshuvas Imrey Yosher 2:80, Chazon Ish O:CH 89:7,11,12 &17, ibid 112:16, Shoney Halochos 358:4.