You see: You duck your head. Others see: <Person> ducks <his/her> head
DUCK (in water) You see: You duck under water for a second, and then come back up, thoroughly soaked. Others see: <Person> ducks down under the water for a second and then resurfaces, thoroughly soaked.
DUCK (in water while hidden) You see: You duck under water for a second, and then come back up, thoroughly soaked. Others see: You hear someone splashing about in the water.