And even without pirates, merchant vessels have incidents with disturbing frequency. A lot of these incidents never have a cause firmly attached to them, because the crew's all dead, the ship is somewhere in a 10,000 square kilometer stretch of ocean, and that ocean is 1km+ deep. Not that the wreck being known and (relatively) shallow is a panacea either, as the variety [0] of hypotheses concerning the cause of the Edmund Fitzgerald wreck should demonstrate.
I trust the US Navy to operate reactors at sea - they've done a damn good job of it over the years, with no losses since 1968. And even those losses (USS Thresher, SSN-593 [1] and USS Scorpion, SSN-589 [2]) haven't caused radiation leaks. Merchant shippers? Yeahhhhh.... No.
0: https://en.wikipedia.org/wiki/SS_Edmund_Fitzgerald#Hypothese...