The three Sand Snakes were there with Elleria watching Mycella's ship sail off with Jamie after she poisoned her. How would two of them then get on the boat? And even if they were on the same boat, wouldn't Jamie have then killed Trystane in retaliation for Mycella's death. And while we're talking about it, when didn't Jamie just turn the boat around as soon as Mycella keeled over to go and tell Prince Doran about what had happened? It wasn't like they were way off at sea.

Man, the Dorne story fucking sucks.