That was very strange. Why wouldn't he speak? If he's heard breathing heavily and later screaming, there's not something physically stopping him from talking, so i reckon he's probably answered the phone without realising. As for the screaming before he went into the water, if there was some kind of struggle something would of been heard on the phone surely? and i find it hard to believe somebody can sneak up on him on the canal as the terrain makes a lot of noise. For me he's simply fallen in.
There's no survivor stories and there's no links to suggest a serial killer. There's been talk of a serial killer for years now so if there was one they would be well aware that killing again in the same manner is going to bring more attention and would have a higher risk of being caught, so why wouldn't they just change how/where they kill? Or if he wanted the "fame" and saw the canal as some kind of calling card they would surely leave more of a link to all their victims.
A lot of the deaths are probably crimes and 1-2 may even be linked to the same person but i don't believe there's a serial killer