To further expand on my previous question and to make it a bit more clear, here is a longer version of it.
As to my question
First the setup.
Ellaria gave Princess Marcella a poison kiss while they were still on land in Dorne, right?
Why did she do that if the Sand Snakes had planned to board the boat regardless? They could have killed Trystane as well as Marcella at once, on the boat.
Your response might be "Because the princess is dead, their guard is down. " In which I would reply
But that shouldn't be true, if your daughter dies in your arms, your guard is going to be up. You will suspect anything and anyone that seems suspicious.
Another response might be "They didn't have enough man power to take on both Trystane and Marcella, plus they'd have to deal with Jamie and Bronn"
That shouldn't be true either, as didn't they setup just last season that Jaime and Bronn were not capable at defending themselves against a couple of Sand Snakes, where in to the point where Bronn got poisoned.
Which further adds to my question.
Bronn is recovering from being poisoned, and Jamie is still not all that great at fighting with his left hand yet, which means that the only person to worry about would be Trystaine, right? And this episode proves that they could handle him on their own.
Not to mention they could have just gotten into their room while they were snogging, caught them totally unawares and dispatched both of them at once.
One further question. Were they on the boat the same time as Jaime? If their goal is to insight a war, why not kill one of them most important people from King's Landing?
Again, yes, there is Bronn to consider, but again, he's in a weakened state. Also, they could have just waited for him to go into a separate chamber if they've been hiding out on the boat for a while.
As for how they got on the boat, it doesn't really matter. The question is why would they poison the princess if they were already planning to board the boat to kill Trystaine?