Plot hole: Patrick Redfern needs someone to be in the boat with him to corroborate his and his wife's deception and be his alibi. That is whole point of his wife lying on beach - he gets this when taking Myra for a ride - but it was portrayed as a random ride she got, he did not intend or plan giving her a ride, he did not see her loitering near the boat, or even in view of him untying the boat. Even if that was intention in the story for him to be seen ready to cast off, the movie viewers did not see her viewing him, only her appearing just before he was set to cast off. Without someone with him the whole deception would not work, he would have no alibi, he could have killed Arlena unseen.
Character mistake: During the surgery, one of the surgeons says to give the patient 25 cc's of cyatril. Then one says make it 35 cc's. 35 cc's (or 35 mL) is a full large syringe. The other doctor uses what looks like a 3 mL syringe and it's only partly filled. It seems the surgeons wanted 0.35 cc's.Bishop73