The Haliplidae, Gyrinidae and Dytiscidae (Coleoptera: Adephaga), or Hydradephaga, of Manitoulin Island, Ontario, Canada were surveyed during the years 1998 to 2003. A total of 2,421 individuals from 86 species were collected from 79 different localities, among which 51 species are newly recorded from that region. This study brings to 88 the number of Hydradephaga recorded in Manitoulin Island. The Nearctic component of the fauna is made up of 76 species (86.4%), while the Holarctic component is made up of 12 species (13.6%). Most species have a transcontinental distribution, but 19 species (21.6%) are recognized as species with eastern affinities. Most of these species are typical of the Canadian Boreal, Mixedwood Plains and Maritimes Ecozones except Platambus semivittatus (LeConte), Matus o. ovatus Leech, Hydrovatus pustulatus Melsheimer, and Laccophilus proximus Say, which stand out as unique Mixedwood Plains Ecozone (roughly corresponding to the Eastern Temperate Forest ecoregion in the United States) species. In an examination of the island fauna, the presence of Hygrotus falli (Wallis), Laccophilus proximus, and Graphoderus occidentalis Horn is deemed notable since they are relatively uncommon in eastern Canada.