The whole miniseries is extremely dark and upsetting- it is always a grey sky or raining, and there are no happy scenes apart from the very last one.
The wrecking and drowning scene is very, very upsetting and hard to watch.
Joss Merlyn is a very intense character, and the way he treats Patience and Mary can be hard to watch.
The whole concept of not knowing who to trust is quite frightening, and the idea of smugglers is upsetting.
A character who is supposed to be good actually turns out to be evil, and a believer in human sacrifice, which is an extremely intense scene, the climax of the series.
A character is mercilessly hanged- you see his lower body and the noose afterwards, and hear very graphic sounds as he dies.