This quest is bugged. I'm not sure if it has something to do with when you find the poison as I found it during my first visit to Thirdborn.
If you pass the +9 Resolve check with Ruanga later in the quest, she joins the captain. But in my playthrough the only conversation option when I return to The Dead Swordfish and take the drinks from the bartender is "[Deliver drinks to captain.] Here they are."
Someone posted a YouTube video showing that the options if you have the poison are supposed to be: 1)+9 Dexterity check option, 2)[Give Kowha and Ruanga poisoned rum], or 3)[Switch cups and give Ngunu and Tauwe poisoned rum.]
Also, I had completed all of the other tasks required to convince Ruanga, and she still wouldn't go unless I passed the Resolve check, too.