Rotterdam Water Police visor cap
The badge and visor edge indicate that this is for an officer in the municipal police (Gemeentepolitie)—as opposed to the state/national police. The shape of the visor has changed in recent years — this particular cap is from the 70s. The golden badge shows that the cap is for the water police and since the only city in the Netherlands that had local water police was Rotterdam, well, that's where this cap came from! Many thanks to Jan in the Netherlands for helping me identify this beauty.