They might not have noticed Nanachi, but Nanachi definitely takes note of them when they come into view. As they look to be a roughly similar age to Nanachi themselves, they aren't as immeadiately on guard as they otherwise would be. Even so, they watch from a distance.

How they play in the water reminds them of a friend they've now been separated from. It stings for a moment before Nanachi squashes it down. Wherever they've found themselves and whatever the trials may be, they'll find their way back to Riko and Reg.

More to the point, the carefree splashing implies that a) the water isn't harbouring vicious creatures waiting to strike or b) it absolutely is and the person isn't aware. If it's the latter... Nanachi can't stand idly by. What would their friends think if they did?

They get close enough to stand by the boots. "Are you sure you should be doing that?" they ask rather flatly.