Anybody can go to a local dive shop or university and get a scuba certification from the Professional Association of Diving Instructors (PADI). You can even get certified at Club Med.
However, that doesn't make you a police diver. Beyond the basic dive certification that most enthusiasts get from the PADI, an agency will probably have to depend on a more specialized source for training.
Dive Rescue International is highly recommended for its focus on training public safety divers and taking its courses on the road. So is Technical Diving International; though most of its courses are geared toward the civilian professional diver, they still teach vital skills needed by the police diver as well. The National Association for Search and Rescue is another teaching organization, though its courses focus is broader.
But there are more resources than you might imagine. The Warwick (R.I.) Police Department's 10-man dive team received training from its area U.S. Navy EOD detachment on how to search for improvised explosive devices. And Warwick PD's accredited unit has also trained teams from departments that are getting off the ground.
The message here is that if you're es-tablishing a law enforcement dive team, don't hesitate to contact an established police dive unit to help you with your training.