Never heard the question before but it's been addressed. Two sources I respect:
https://what-if.xkcd.com/15/
https://www.quora.com/If-a-nuclear-b...-at-that-depth
What I can't find is any mention of the underwater earthquake triggered off Fukushima by a hostile nation using a nuke pilfered from a sunken submarine (the Kursk?). Down the memory hole.