Most navies (except the USN) have diesel subs for use in coastal waters. A diesel is inherently quieter than a nuke because you can turn off all of the machinery in a diesel sub (except for the electric motors) and run silently. On a nuke you need to keeps pumps running to cool the nuke plant, and coolant circulating creates a lot of noise (relative to that of an electric motor slowly turning a propeller).
There are subs that use nuclear power plants that can operate with natural circualtion, the main coolant pumps are only required at higher power levels. The USS Narwhal used natural circulation and I believe the S8G reactor (Ohio class)design also uses natural circulation. I served on S5W reactor plants so I don’t know if the main sea water pumps are required (to condense steam) when the reactor plan is operating in natural circulation.