In the Things Betwixt zone, what does the coffin by the lake do?
Solution 1:
The boat changes the sex/gender of your character. If you enter it as male, you come out as female and vice versa. It doesn't change anything else about your character, so any hair/makeup/tattoos/body style/etc remain as before.
Since so much of your body shape is obscured by armor, the change isn't very obvious. Additionally, there are absolutely no known gameplay differences between playing as male vs. female, in the base game. If you have the DLCs though, there are certain armor sets that will give you hidden benefits if you're a certain gender.