There is a very tiny effect on one’s weight from the lunar tidal force, but…this effect has absolutely nothing to do with the chemical composition of the human body. The peculiar idea that tidal forces have some kind of special affinity for salt water is completely false. Like any other gravitational effect, tidal forces affect mass, independent of its chemical composition.
We recently ran a show performed years ago in Salt Lake City (SY-424 for those of you who collect and archive). Richard asked Paula why we are all heavier at low tide… Dave Redell offered this salty tidbit: