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.
Easiest and safest way to tame this is to lure it under water. They stop aggro and you can tame as you please. Just make sure you come up for air every once in a while. Also be sure the water is deep enough.
arthros are really easy to tame if you have a pulmonoscorpius, it grants you the stealth needed to do the passive tame (i think that araneos do that too but they're harder to catch and their saddle unlocks way later). you can just search the highest level arthro in the cave and stock up on rotten meat while doing so, just keep in mind that the saddle unlocks at level 54 and there's not much of a point in keeping one before having a saddle