I think the reason some people might believe this claim is because we’re taught in school that the moon’s gravity causes the tides. I think the reasoning goes, “well if the moon’s gravity can affect the tides, surely it can affect smaller things too”
https://oceanservice.noaa.gov/facts/gltides.html
If the great lakes only see a fluctuation of 5cm max, I don’t think it is affecting the human body in any meaningful way.