According to Apple's website: "Classic Buckle, Leather Loop, Modern Buckle, Milanese, and Link Bracelet Bands aren't water resistant." There are many possible reasons for this, but here's one explanation I found from a forum: "The weakness in the milanese loop band is where the magnet is embedded. The magnet is made with iron and is not in a waterproof compartment. It is simply glued into the pocket and covered with another glued on piece of shiny metal. When water gets into this area, the magnet rusts and expands pushing out the the shiny metal plate until it extends beyond the grippy rubber ring. At this point the magnetic latch doesn't work very well anymore because the rubber no long grips the band and the magnetic latch easily slides open."