I just reread and it is not awfully clear if Eb saw Murphy in the boat. She was on the wheel. Eb has to know that someone was driving (not Lara, as he saw Lara with Harry). I think he also saw Freydis. So perhaps he thought someone working for Lara was on the wheel and he chose to think it would be not a mortal. But it is not too clear, because there is a moment when Harry saw Murphy nodding to Lara and I think at that moment she was visible to Eb. That said, I am not sure. I would have liked that when Eb said "I'll sink the boat", Harry (or even Lara) would have told him "Are you sure? There are mortals inside"