Extremely minimal plot details, in fact.
Right, this may or may not have been made for television. Television is where I saw it at some point in the mid to late-ish 1980s (which doesn’t necessarily mean that is when it was made.
There is a couple on a canal boat (on a holiday trip, I think, rather than permanently living) and they take aboard another guy - perhaps his boat has broken down or something. Sex and violence ensue (wife fascinated by this stranger, that sort of thing.) Not the big loud explosion type of violence - more like sneaky human stuff.
I seem to recall there’s a bit of tying people up in dark cupboards - either wife or husband or possibly both at different times. I do not recall if anyone got killed, but I recall it all being pretty dark. Well, it seemed a bit scary to me, but scaring me isn’t difficult.
I recall some phrase like “Limit of Navigation” which I thought must be the title, but it does not seem to be.
What else can I think of? Let’s see. I do not have the impression that it was ever a particularly famous film/drama/whatever, but I could be wrong about that. No especially famous actors, as far as I can recall.
I don’t know why I want to know about this. Really, it is just the maddening annoyance of a bit of useless trivia that I cannot recall. For some reason, I just Googled it, you see, and the only reference I found was someone else asking for identification of what I am sure is the same film. Haha, the answer given to that person involved an Alan Ayckbourn “comedy-horror”. Nope, I don’t think this can possibly have been Ayckbourn, and I don’t recall any comedy.
(I found it quite funny that just when one assumes that Google will find anything, all I could find was someone else looking for the same thing).
So, asking for no really important purpose, and will very little hope of an answer, but I figure that the Dope has plenty of other people who hate it when they forget a title or writer or whatever, so it’s worth a try. 
