I should also add they don't always mean instant destruction. However, they do result in serious damage to the hull.
I've survived a FIDO hit. But the problem is dealing with future aircraft on the way back to base with a seriously damaged hull.
This usually results in a sunk boat at some point unless very lucky. Best to travel submerged all the way back, but there's that pesky problem of having to surface to recharge the batteries that can cause trouble......
