What underwater detection devices are used in the military?
underwater listening devices the navy uses sona bouys which is an active systems that is place in a pattern where suspected submarines are, they are launched out of a p3-orion or an S3-viking, also they have a system of underwater microphones called the sousis (maybe not the correct spelling) but it monitored 24/7.
Also, most ships and submarines in the Navy have passive SONAR (listen only mode) as well as active (send a sound, then listen for the return) to detect what is near them in the water. The advantage to using passive, is that if you can keep your own ship quiet enough, the other vessel may not know that they have been detected.