I've found that it's 50/50 for me.
Just 2 hours ago (literally) I was approx. 750 km due west of Gibraltar. Convoy contact was given on the map. Luck of the draw! It was headed my way, as I was headed there way! yay!
Okay, weather was: C R A P! foggy, rain, 15 meter/sec wind. Okay, they won't see me, and I'll have a heck of a time seeing them.
So I dived. I put myself at 150 meters down. Any further and I start to get damage (I'm not using the crush depth mod). Shut the engines down, rig for silent running and just sit there, hoping that the convoy doesn't shift or that we run out of air!
Again, I'm in luck. Here they come, sailing along as though nothing..........ah crap.
The lead escort suddenly starts turning his screws like he's doing a power run! .
.
. <snip>
.
So here he comes, right to where I'm sitting. Okay, what did I do wrong? I'd had no contact with anyone in 3 days. No ship sighting in three days. I've kept radio silence since the patrol began. I dove down to my present depth of 150 meters when the convoy was still over 50 km off. Sea state is at least a 6 or 7 up there. Okay, maybe his sonar man has ears of gold on that DD and he heard me sipping my coffee from 15 km away at 150 meters. As Darth Vadar would have said: "Impressive."