you are correct, its the lifter cover gasket. however that would not cause rust streaks. only a cracked head from improper winterization would cause rust streaks on that side of the motor.It’s a 2009 bayliner so it is strange, that isn’t a crack it’s a plate and gasket than can be replaced apparently.
But I am concerned if that rust has gone further. Only once on sea trial it appeared to be worse.
Thanks