Is a person considered to have performed ablution simply by diving into a swimming pool with the intention of performing ablution? And what is the reason for my father's diabetic retinopathy not being cured by ruqyah despite repeated attempts?

Swimming in water is sufficient for ablution (wudu') if the person intends to remove ritual impurity, and the water reaches all the organs of ablution, and the organs are washed in the correct order and rubbed, by scholarly consensus. However, if the correct order and rubbing are not achieved, there is disagreement regarding the validity of the ablution. The Shafi'is, Hanbalis, and some Malikis made ordering obligatory, citing the verse in Surat Al-Ma'idah which places the wiped (feet) between the washed (face and arms). Whereas the Hanafis and Malikis considered it a sunnah (recommended) and not an obligation, because the waw (conjunction "and") in the verse does not necessarily imply order. As for rubbing, the majority of scholars believe it is not obligatory and that the water reaching the organs is sufficient, while Malik and Al-Muzani stipulated it. As for ruqyah (exorcism/spiritual healing), those who practice it must persevere with it, along with sincerity, avoiding sins, and frequent supplication, without haste.