I think it would be more helpful to say that damage must be equal to or greater than 70% in order for a vessel to sink.
However, this subject is frowned upon by the majority of forum members for one good reason.
Sinking vessels are in no way fun.
The closest I have been to being on board a sinking ship was in a naval simulator, and that scared the living bejesus out of me.
EDIT: Telling a moderator to shut up will get you banned. Actually, telling anyone to shut up will get you banned.