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.
Other mistake: Poirot in his summing up of what took place in the murder of Arlena, makes many assumptions about exactly what took place on the beach, he said (in so many words) - Arlena went to the pedalo in order to leave the beach on it, but then ran in to cave instead. How could he possibly know this? There were no witnesses or evidence of this. All he knew was she must have gone into the cave as the gem stone was in there.