Only today I was diving, trying to evade a destroyer. I realised I was in fairly shallow water and "pinged" to see how deep it was. The little submarine(the noise detector-goes red when you're in trouble)went red and the destroyer made a bee line for me.
I say that I was about 2k away from him and fortunatly I did manage to shrug him off.
Good job I did ping, there was only about 40m under me
