Well…actually she did do that already from when Yuri first get taken as a kin.
Remember? Pulled up her shirt and got sucked from her chest and everything!
I'm thinking they might not actually be on an island, just further down the beach from where they started. That would be a funny gag. Ruruna is an idiot and Yura is sick so there is nobody with a functioning brain to properly assess the situation.
I think Ruruna is going to fly off the island. I know it's not happening, but I kinda hope she'll turn into Coppola's Dracula-style eldritch giant bat-monster-thing to do it, which will give everyone (including Wakana) second thoughts.