From Bulbapedia, the community-driven Pokémon encyclopedia.
The Water 1 Egg Group (Japanese: すいちゅう1 Underwater 1) is one of the fifteen Egg Groups. Like its counterparts Water 2 and Water 3, it is mostly made up of Water-type Pokémon. As of Generation VI, 89 Pokémon belong to this group.
Water 1 group Pokémon tend to be mostly terrestrial Water-type Pokémon, with few fishlike Water types among them. Most are capable of both land-based and aquatic locomotion. It can be thought of as being similar to the Field Group in that it is a "catch-all" Egg Group for aquatic and semi-aquatic animals.
The majority of Pokémon within this Egg Group are capable of breeding with Pokémon of other Egg Groups.
Only in this Egg Group
In this and another Egg Group
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