With regard to the mental stability of someone working in submarines (indeed one could argue anywhere in the military) there was that tragic case back in 2011 when a submariner on armed guard duty ran amok and killed one and injured several others of his crew:
Submariner's grudge turned proud day into tragedy - BBC News
What the RN did as a result of that incident I don't know, but perhaps this latest incident may be another wake up call regarding monitoring the mental state of those serving?