D&D 5E - City Watch Investigator Background
Waterdeep, Skuld, Calimport, Gheldaneth , Unthalass, Suldolphor, Bezantur, Eltabbar, Athkatla, Zazesspur and Cimbar all have over 100k populations, so I guess they all could have the requisite level of Watch. But whether they have the right culture to produce 'gumshoes'....