That’s like saying I’m only responsible for not running into the car in front of me, and not responsible for watching the guy next to me to be sure he doesn’t swerve. I mean, yeah, if he does swerve and hits me, it’s his fault, but had I been paying attention I might have been able to prevent the wreck in the first place, and it’s my duty to do that.
Good seamanship, like good driving, dictates that you know what’s going on everywhere around you, not just in front. That destroyer is like a speed boat compared to the tanker. If they’d seen the tanker, they should have been able to get out of its way, barring more traffic blocking them in. The question is why that didn’t happen - is it because no one was looking, or were they boxed in?