Answer
✅Answer: Please pay attention to the following narrations:
1 عَلِيِّ بْنِ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنِ ابْنِ أَبِي عُمَيْرٍ عَنْ حَمَّادٍ عَنِ الْحَلَبِيِّ قَالَ سَمِعْتُ أَبَا عَبْدِ اللَّه يَقُولُ إِذَا ارْتَمَسَ الْجُنُبُ فِي الْمَاءِ ارْتِمَاسَةً وَاحِدَةً أَجْزَأَهُ ذَلِكَ مِنْ غُسْلِهِ.
📚 Al-Kafi, Vol. 3, p. 43
Halabi says: I heard Imam Al-Sadiq (peace be upon him) say: 'If a person in a state of major ritual impurity (junub) submerges himself in water once, this suffices for his ghusl.'
2 عَنِ النَّوْفَلِيِّ عَنِ السَّكُونِيِّ عَنْ أَبِي عَبْدِ اللَّه قَالَ: قُلْتُ لَهُ الرَّجُلُ يُجْنِبُ فَيَرْتَمِسُ فِي الْمَاءِ ارْتِمَاسَةً وَاحِدَةً وَ يَخْرُجُ يُجْزِيهِ ذَلِكَ مِنْ غُسْلِهِ قَالَ نَعَمْ.
📚 Al-Kafi, Vol. 3, p. 22
Sakuni says: I said to Imam Al-Sadiq (peace be upon him): 'A person becomes impure, so he submerges himself in water once and comes out. Does this suffice for his ghusl?' He said: 'Yes.'
3 مُحَمَّدُ بْنُ عَلِيِّ بْنِ الْحُسَيْنِ بِإِسْنَادِهِ عَنْ عُبَيْدِ اللَّهِ بْنِ عَلِيٍّ الْحَلَبِيِّ قَالَ حَدَّثَنِي مَنْ سَمِعَهُ يَقُولُ إِذَا اغْتَمَسَ [إِرْتَمَسَ] الْجُنُبُ فِي الْمَاءِ اغْتِمَاسَةً [إرْتِمَاسَةً] وَاحِدَةً أَجْزَأَهُ ذَلِكَ مِنْ غُسْلِهِ.
📚 Al-Faqih, Vol. 1, p. 86
Halabi says: 'A person who heard from him (i.e., Imam Al-Sadiq, peace be upon him) narrated to me that the Imam said: 'If a person in a state of major ritual impurity submerges himself in water once, this suffices for his ghusl.'
4 عَنْ زُرَارَةَ قَالَ: سَأَلْتُ أَبَا عَبْدِ اللَّه عَنْ غُسْلِ الْجَنَابَةِ.... إلی أن قال وَ لَوْ أَنَّ رَجُلًا [جُنُبَاً] ارْتَمَسَ فِي الْمَاءِ ارْتِمَاسَةً وَاحِدَةً أَجْزَأَهُ ذَلِكَ وَ إِنْ لَمْ يَدْلُكْ جَسَدَهُ.
📚 Al-Tahdhib, Vol. 1, p. 148
Zurarah says: I asked Imam Al-Sadiq (peace be upon him) about the ghusl for major ritual impurity; until he said: 'And indeed, if a person in a state of major ritual impurity submerges himself in water once, it suffices for him, even if he does not rub his body with his hand.'
5 عَنِ ابْنِ مَحْبُوبٍ عَنْ مَالِكِ بْنِ عَطِيَّةَ عَنْ أَبِي حَمْزَةَ عَنْ عَلِيِّ بْنِ الْحُسَيْنِ قَالَ: لَا عَمَلَ إِلَّا بِنِيَّةٍ.
📚 Al-Kafi, Vol. 2, p. 84
Imam Al-Sajjad (peace be upon him) said: 'No action is (valid and acceptable) except with intention.'
✅The above narrations indicate the validity and permissibility of the immersion ghusl.
✅ Intention, as a necessary condition in most actions and acts of worship, is also necessary for the immersion ghusl. This means that a person who intends to perform ghusl enters the water completely with the intention and purpose of ghusl, according to the method that has been narrated from the Ahl al-Bayt (peace be upon them).
✅ According to the apparent meaning of the above narrations, the certain way to perform a valid immersion ghusl is for the person, with the intention and purpose of performing ghusl, to fully enter the water. The intention to perform ghusl after entering the water or at the time of exiting the water is not certain. Therefore, a person who is inside the water and decides to perform an immersion ghusl should exit the water and then enter the water completely once with the intention of ghusl, and this will suffice for his ghusl.
✅Please also note the following narration:
6 عَنْ يَعْقُوبَ بْنِ شُعَيْبٍ عَنْ أَبِي عَبْدِ اللَّه قَالَ: لَا يَرْتَمِسِ الْمُحْرِمُ فِي الْمَاءِ وَ لَا الصَّائِمُ.
📚 Al-Kafi, Vol. 4, p. 353
Imam Al-Sadiq (peace be upon him) said: 'The pilgrim in ihram and the fasting person should not submerge in water!'
✅ According to the above narration and similar ones, because a pilgrim in ihram and a fasting person should not submerge their heads under water, performing an immersion ghusl is forbidden for a fasting person and for a person who is in the state of ihram.
✅ What is mentioned in the narrations of the immersion ghusl is complete submerging in the water, and there is no difference between entering gradually or entering all at once (diving). Also, it does not matter which part of the body enters the water first.
✅ You can see the method of sequential ghusl in the following link:
اللَّهُمَّ فَصَلِّ عَلَی مُحَمَّدٍ وَ آلِ مُحَمَّدٍ وَ عَجِّلْ لِوَلِیِّکَ الْفَرَجَ وَ الْعَافِیَةَ وَ النَّصْرَ