Walrein locations — where to catch

In Hisui (Legends: Arceus)

• Ginkgo Landing — Walk
• Hideaway Bay — Walk
• Islespy Shore — Walk

Other games (3)

• Sword / Shield — The Isle of Armor (Gen 8): Frigid Sea, Giant's Bed
• Black 2 / White 2 (Gen 5): Undella Bay
• Black / White (Gen 5): Undella Bay

Walrein — full Pokédex entry