I don't see a national conspiracy either, but I think one (or perhaps two) people are responsible for most of these deaths. I don't think a gang could have stayed under the radar for so long.
I followed Brian Welzian's missing person case in 2000. When he was found in Lake Michigan, I didn't have any reason to doubt the accidental death theory. But within a year or two, there were several similar cases in the Midwest and I began to suspect a connection between the cases. Later I found out there were several cases before Brian's death.
I don't necessarily buy the smiley face connection as smiley face graffitti is found in many places.
Besides the similarities of the circumstances of the disappearances, the bodies being found in water, and many of the bodies in better shape than they should be if the victims had fallen in the water the night they disappeared, there are two reasons I believe at least some of these cases are connected.
1. The fact that young men apparently weren't falling in the water and drowning in such high numbers before 1997. From about 1970, I was in college for 4 years, at a college for 3 years while my husband was in grad school, and then for years at several universities where my husband was a professor. Many of those years were in the Midwest (Michigan and Illinois). I don't recall any such deaths back then.
2. The geographic concentration of these deaths. Are Midwestern college boys dumber than college boys in California or Florida or non-Midwest college towns situated on a river, lake, or coast? I don't believe so. What can explain the cluster of these deaths in the Midwest?