With an intact tympanic membrane (ear drum), the increasing water pressure against the ear plug and the decreasing volume of air between the plug and the tympanic membrane tend to drive the ear plug against the tympanic membrane. If the plug wedges within the ear canal, there is risk of external ear canal barotrauma.
Plugs that have a hole in them allow water to enter the canal and defeat the purpose of the plug. If there is a perforation of the tympanic membrane, this could cause middle ear infection. Advocates of the use of ear plugs have not presented enough evidence to recommend the use of plugs.