I was a kid when I saw it so it must have been in the 80's, I think it was a family or a group of friends that have hired a riverboat or maybe two for a trip, as they go further up river thing get stranger and stranger, I think people got murdered, at the end on of the cast jump out of the water all covered in mud and maybe turned into a monster. My memory of it is so vague it makes me thin it might be early 80s when I was 5-7. I think it might have been part of BBC Screen One or something similar, it was a one off drama sort of thing.