Kim, hate to break it to you... but Sora's not being accurate. There's a reason why nekovalkyrja stealth is called "thermoptic camouflage" - it helps hide the neko both in the visual and infrared spectrum.
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Raz, the sword couldn't be more than a few meters away, but it also drop to 'the bottom', where I previously established there was a stand of... y'know, kelp-equivalent stuff.
If Saiga wants to look for it, then go back and help Kotori, the 20 second estimate is generous. If you include the water's drag and the time to look for it, yeah, it won't be immediate.
That doesn't make Nicholas unable to help if he wants to, though. He still has that Type 28 SMG, doesn't he? That thing's an energy weapon - it'll have less range and accuracy, but it's going to work underwater.