Presumably the reactors in submarines are commanded to scram when the sub is in trouble. So the reactor is a high corrosion resistant stainless vessel in non-c

Presumably the reactors in submarines are commanded to scram when the sub is in trouble. So the reactor is a high corrosion resistant stainless vessel in non-critical state at the bottom of many tens (hundreds?) of metres of ocean. All told, that doesn't seem an entirely unreasonable place for it. Even if it was critical, in the active state before the sub sank, it'd probably just heat water for a decade or too before it eventually cools down. Even if it suffered a meltdown, that's not ideal, but I'm not gonna lose any sleep over it.