The rumored submarine accident is detailedly reported by DailyMail.

Will China admit the fact that a submarine incident happened and killed some crews?

https://www.dailymail.co.uk/news/article-12589429/chinese-sailors-trap-yellow-sea.html

If any Chinese state owned/controlled media outlet report the incident as fact or Chinese centrsl government confirms it, this resolves to YES.

The detail of incident can be different. The important part is that a submairne accident killed some crews under water and it has been hidden from the public for some time.

55 Chinese sailors feared dead after submarine 'caught in trap'

EXCLUSIVE Officially, China has denied the incident took place. It also appears Beijing refused to request international assistance for its stricken submarine.