Walaikum assalam wa rahmatullahi wa barakatuh,
I pray this finds you in the best of health and spirits.
The ruling of watching boxing would be derived from the ruling of boxing itself. Boxing is Islamically problematic--as professionally-practiced--because it involves striking the face, which is firmly prohibited in Islam. The Messenger of Allah (peace and blessings be upon him) forbade hitting even animals on the face.
[ref: Khadimi/Birgivi, al-Bariqa al-Mahmudiyya Sharh al-Tariqa al-Muhammadiyya]
And Allah alone gives success.
