The reasons why the Nimrod is useless at Anti Submarine Warfare are down to the laws of physics.
Consider a Nimrod flying along at 200 feet. It fires a sonabouy out the back into the sea. These things are pretty heavy and by the time it hits the water we're talking about a lot of kinetic energy here.
The submarine CAN HEAR the sonabuoy hitting the water with its sonar, and calls a 'splashdrop'. Not only that, but it can also get a range in yards and a bearing to the sonabuoy.
Its not rocket science....!