Where to Find and Catch Walrein
From Pokemon Go Wiki
Walrein are found and captured in Nest and Habitat locations.
Nests
Walrein Nests are locations that consistently spawn Walrein.
Habitats
Walrein Habitats are spawn locations where Walrein is likely to be encountered, but not guaranteed the way a Nest is.
Additional Tips and Information
Best Nests
If you know a really good nest or any of the nests above is really good, list them here: