-( Story related, cannot be missed )-
After rescuing Fritz, you'll start fishing and one fish will have a bottle with it. It'll be from Cecille and you will then be tasked with finding the last three crew mates. They'll be located at each of the remaining three islands (not including the far east one). Brenda, Teresa, and Cecille are your targets. Brenda will be located in Sou Island (south island, just giving it a fancy name); Cecille is located in Souwes Island (southwest island, rather deep in by her makeshift camp); and Teresa is on the platform in Norwes Island (northwest island, you'll eventually see that she's near a house after exploring the pier). By talking to them, they'll request you take them to everyone else, so make sure you've cleared enough for your first trip before talking to them and taking them home.