In GWX 2.1, the recognition manual shows where the weak spots of the the ships.
It happens (at least to me) at least 450m and near perpendicular (at almost 90 degrees) postion from the target!
But in my tactics, I try to aim for the bow! Someone on this forum (can't remember who) does bow shots and the momentum of the ship brings more water in so the ship sinks itself! Cool, huh?