AFAIK a sunken reactor is not as big of a threat to life as a one (marine or land-based) that releases nuclear material in the atmosphere, so the biggest issue should be what may happen before the reactor sinks.
Anyway IMHO the biggest issue with nuclear is not its safety, but rather that, even when it operates without the slightest of incidents, it produces waste that needs to be kept “safe” for periods of time that exceed the age of most nation states (let alone private companies).