So, I've looked it up, apparently there's a hint at the drying hut that points toward Rongi. I couldn't find it. I ended up reading Rongi's journal, and it tells me Rongi can be found on the beach at midnight (like what was posted above), I wait until midnight, and Rongi just chills out at his hut, and I can't progress his conversation any further than "What do you do here?" after the initial options.