These people were trying his patience. He glanced around at every single one of them, glaring though they couldn't see him. Feelings and fears and whining wouldn't get them anywhere. In a small, human portion of his brain, he felt bad for Nick. He didn't want to upset him. But coddling him wasn't going to do anybody any kind of service, either.
When all three of them disappeared beneath the surface, Archer looked up to avoid watching them descend. He didn't want this whole trial to be invalidated simply because he saw the hiders hiding and "cheated." As he slowly counted to seven, he unbuttoned his pants and shrugged out of them, kicking them off. After removing his socks, he was completely invisible. Save for the slight frothing of water around his feet as he kicked to stay afloat, nobody would have known he was there.
Once he reached "seven," he took a deep breath and dove. Though he was out of shape, he was able to swim well enough. As he neared the bottom, he saw a tuft of dark hair peering out from behind the plastic plant nearest the castle. He recalled the blonde demanding that he find Nick first, and though he couldn't be certain of this person's identity, the length of the hair suggested that it wasn't the other woman.
Archer made a beeline for that plant, feeling his feet brush the gravel as he reached the bottom. To avoid terrifying the already scared boy and cause him to inhale two lungs' worth of water, he picked up a piece of pink gravel and held it up. He walked towards the boy, waving the piece of gravel before his eyes. When it was clear that he had seen it, Archer reached out and touched the gravel to his arm just above his elbow. To ensure once again that they wouldn't be forced to do this again on a technicality, he also turned his hand so that he could tap two fingers against Nick's arm. Once that was done, he threw the piece of gravel in the air, hoping that the boy would understand the signal to return to the surface.