Anas bin Malik RA reported:
"The hair of Rasulullah (SAW) reached till half of his ears."
Aishah RA reported:
"Rasulullah (SAW) and I bathed in one utensil, and the mubarak hair of Rasulullah (SAW) was longer than those that reached the ear lobes, and were less than those that reached the shoulders."
(It means that they were not very long nor were they short, but of a medium length.)
It is not clear from the second aforementioned hadith if both bathed in a state of undress. Nevertheless, the etiquettes of the Sahabah RA in every affair of their life was exemplary for the Ummah to follow.
Aishah RA herself said:
"I did not see the private parts of Saiyidina Rasulullah (SAW), nor did he see my private parts."
Also the using of water from the same utensil cannot prove this. There are many ways both could have bathed from the same utensil and yet not see the private parts of one another.
From this hadith we also learn of the bathing together of husband and wife. The Ulama are unanimous in accepting the saying of Imam Nawawi RA that this is act is permissible. Another method is that the husband bathes first and the wife bathes after him with the remaining water. This is also unanimously accepted by all. The third method is the opposite where the wife bathes first then the husband bathes with the remaining water. This method is accepted by the Hanafis, the Shafi'is and the Maalikis. The Hanbalis says it is not permissible (of which its explanation from the dynamics of their system of juristics justify - but that is not mentioned here for the sake of brevity), but if the husband is present and both bathe together, then it is permissible. As a whole, the pros and cons of this subject are very lengthy, therefore they have been omitted here. However, let us take the stance that - since a reputed Imam (i.e. Imam Ahmad bin Hanbal RA) opposes this, and this has also been mentioned in the hadith, it will therefore be better to abstain from such practices.