When dropping a transparrent Minecraft item or block into water it vanishes however, when it floats up the bit floating above the water is still visible. This may be to do with overlaying and rendering.
To recreate this go to snapshot 19w45b and throw a transparrent item/block underwater e.g. ice, glass. (works in all gamemodes that can throw blocks and items.)
video proof here \/
MC-165156 Transparent block items no longer render behind transparent blocks