That was an awesome post, Zack. Very descriptive!
So, the characters are currently at the junction or crossroad of an underground canal system. They're on a small slab of concrete about four by four meters, accessible only be swimming. I would say, that Rarmon has found the junction but has traveled via another passage which is above the water level.
To fit your passage, we can write that the characters on the slab will see him approaching through an open passage way, about ten meters away across the water. That would work out nicely so that we don't have to swim through the canals, although Plymuth probably wouldn't mind it. Taking the passage way back would ensure that they do not run into any of the giant rat-things.
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