How far is the distance of sonar detection?

Some locations in the ocean are much noiser than others due to depth, proximity to shore, traffic, and biological activity. In passive sonar vessels are tracked and identified by their sound emissions. For submarines there is a lot of effort made to mask the sounds emitted to make tracking and detection difficult esp. at distance. read more

So detection depends on the design of the vessel, the speed of the vessel, the location of the vessel (ocean conditions), and the sophistication of the sonar boats electronics and noise reduction. In some texts they show detection open ocean distances as much as 1000 km for older sub technology hundreds and down to 5 km for late model subs. read more

One should note that some diver detection sonars only provide specifications for the best conditions, not for the worst ones. read more