If there was some serial killer or killers dispatching these young males, don't you think at least one of them would have escaped or they would have found wounds or lesions on one of the victims indicating a struggle? Also nobody heard any cries for help. Just the odds of some lunatic being able to kill so many young strong people, however inebriated, without signs of a struggle makes me doubt the whole serial killer paradigm.
p.s. They've pulled a few people out of river before they drowned since the onset of the drownings, and one lucky idiot managed to get himself out of the water after falling in. He had no idea how he fell in. He only remembered drinking heavily and wandering away from the bars. He also added that no one pushed him in.