Be aware, as a former submariner, one of two actual "emergency deep" manuvers I was involved in was on the account of a sailboat. Submarines are everywhere and they use the accoustic signature of all ships, boats, submarines and beasts of the sea to determine what's out front, in their path. This usually works out well on vessels turning screws, not so much those without.
Sailbaots don't have a very large radar
signature either, reflectors help. I'd assume the vessel in question had a small or zero radar