The problem with ocean mines is proving who put it there.
The yield required for a deep-ocean mine to have lethal effects through 10,000+ feet of seawater would rule out all but a few nations--actually, it would narrow it down to two, because we can rule out the US in this scenario. Russia and China. Russia would not be overly pleased to see China gain hegemonic status in the Far East, because the ChiComs might start getting ambitious re: Siberia. That leaves China.