Where to Find and Catch Corphish
From Pokemon Go Wiki
Corphish are found and captured in Nest and Habitat locations.
Nests
Corphish Nests are locations that consistently spawn Corphish.
Habitats
Corphish Habitats are spawn locations where Corphish 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: