Is it permissible to perform ghusl from janabah in a swimming pool merely by immersing the body in it, taking into consideration the presence of added substances, dead insects, and dust in the water? And does this affect the purity of the pool water, thereby preventing ghusl in it again?

It is permissible to swim in a pool if it contains more than two qullas (a measure of water). Ghusl (ritual bath) performed in it is valid, and major ritual impurity ( janabah) is removed. However, if the water is less than two qullas, swimming in it is disliked according to the majority of scholars, and the water becomes musta'mal (used), and janabah is not removed by it according to some scholars. As for water that has changed due to pure substances or dead insects whose death is pure, it is not suitable for purification according to most scholars, while it is valid according to the Hanafis and in one narration from Imam Ahmad and Ibn Taymiyyah.