hmm interesting to see you got spotted at that distance.
I conducted my first GWX surface attack on a lone passenger/cargo on a patrol last night - dead calm sea, clouds in the sky but still with moon. I had setup my approach before hand though, so I was perpendicular to him and in place well before he arrived - I was sitting completely "dead" in the water with zero speed. He passed my bow at about 700 meters and never spotted me.
He knew I was in the neighbourhood after receiving an eel though
Unfortunately, as i'm still experimenting how to get a one shot kill with this type of boat, the first shot didnt sink him, so I needed to come about and make another shot at him. I noted as soon as started with forward speed and turned parallel to him (obviously creating a mech larger sillouette) he spotted me. Then I submerged just to be nasty and put him out of his misery hehe.