I just tamed one of these on Crystal Isles at Eldritch Isle. They don't go passive in the water immediately, but I threw one in with my Argentavis, went into the woods to unrender the area I threw the Arthropluera, and when I came back and dove into the water, she had chilled out, and I was able to feed and tame it. I'm on 10x taming, so Idk how long the passiveness would last after the first feeding, but THAT'S how it works; it's not just magically passive just because it gets wet.

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