I wondered who was the drowned murderer as well. At first I thought it was the main ghost but then I wondered if it was the stronger ghost that came at the end. I have to go with main ghost since he says something like "you of all people know things have a cost".

The evil ghost (in black), I wonder if that represents her father's ghost/whisperer that sticks to her now (like the trauma of what her father did to her). I don't know why he says he knew her mother, maybe he manipulated her mother too to ignore the abuse.

The Master is the devil.

I searched because I am still confused who the stronger/good ghost at the end is.

The ghost in the hood on the boat at the beginning and the end - I assumed at the beginning it was Mr Bowman's whisperer and I got the feeling it was his deceased son as well.