What are these nymphs doing?

[MarcoP]

There's something strange though: a clear pen line in the water; what could it be?

Absolutely no idea.

Anyway, the pcholshedy pool in You are not allowed to view links. Register or Login to view. is matched by a slower polshy choreography in the other side of the bifolio (f75r). Here we have a couple, a triplet and an out-of-scale, possibly more prominent, giant nymph.
In all these three triplets, the nymph at the back seems to be grasping the forearm of the nymph at the center, who is touching/holding the nymph at the front with her other arm.