Purity: Instantaneous Immersive Ghusl (Irtimāsī Dafʿī)
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 366:
Two types of immersive ghusl can be performed: instantaneous (dafʿī) and gradual (tadrījī).
In instantaneous immersive ghusl, water must cover the entire body in one go. However, it is not necessary for the entire body to be out of the water before starting the ghusl; rather, it is sufficient if part of the body is out of the water and the person goes under the water completely with the intention of performing ghusl.[1]
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)
An example of instantaneous immersive ghusl is when a person, with the intention of performing ghusl, dives/jumps into a swimming pool and in doing so completely immerses himself in the water; or, when a person who is already partially immersed in the water completely immerses himself with the intention of ghusl.