Question Summary:
1. Can I perform ghusl (bath) by just letting the shower flow without doing wudu if I am not going to perform Salāh? 2. I performed ghusl. After Salāh, I witnessed semen. Do I have to perform my Salāh over again?

Question Detail:

1. Can I perform ghusl (bath) by just letting the shower flow without doing wudu if I am not going to perform Salāh?
2. I just did my ghusl and prayed. However, after my Salāh I saw semen. Do I have to wash myself again and repeat the prayer or is my prayer valid?

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful. 
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
There are three necessary acts in ghusl (bath):
1.      Rinsing the mouth.
2.      Passing water into the nostrils.
3.      Passing water over the entire body.[i]
If any one of these acts are left out, the ghusl will be incomplete. However, one should assure that the water reaches every part of his body. Making wudu is not a condition for the validity of ghusl.[ii] Therefore, one can also perform Salāh with the ghusl in which he did not perform wudu.
Regarding your second question, if you witnessed semen after your Salāh, then
ghusl will become fardh upon you but repeating your Salāh will not be necessary.[iii]
And Allah Ta’āla Knows Best
Mawlana Abdul Hannan Nizami,
Student Darul Iftaa
USA
Checked and Approved by,
Mufti Ebrahim Desai.
www.daruliftaa.net