AJ peered over the edge of the glowing water. The spotlight of the moon glittered prettily, although the surface of water was disturbed as a face appeared, breaching the shallow waves.

"Are ya... sure ya don't need anythin' else, Ra?" AJ asked, her eyes darting down the length of Coloratura's tail.

Rara shook her head. "This water and your smiling face is all I need to calm me, AJ." Rara beamed up at her marefriend, feeling her gills flutter with each breath. Breathing became easier in the water at night when she swam first.

AJ admired her marefriend. The country pony had seen her in this form before when she had reacted to the full moon a few weeks prior. A difficult conversation ensued between the two, but one that led to an understanding. Rara never meant to hide her identity from AJ as a means of deception, but it stemmed from fear of the pony population as a whole. Rara had already lost her sisters by pony hooves. She did not want to lose AJ as well.

So now the two sat in comfortable silence, a siren and a pony, enjoying the water and the moonlight as they listened to the crickets and the howl of the distant timberwolves

SO AJ did not take Coloratura's first full moon when they started dating very well. AJ is very triggered by... full moon events... and so finding out her girlfriend actually swims in a lake at night as a big scary monster was... terrifying. 

AJ asked if Rara meant to  kill  her and Rara realized AJ had some DEEP trauma in that area... There was a lot of reassuring that Rara never meant to hide her form as a malicious thing, just out of fear for her own wellbeing.

The two healed and then the next time Rara needed to dip into the water to refresh her breathing and relax her muscles, AJ offered to let her use the old watertower that sits in the field and is never used. From then on, that was Rara's watertower. (Also no one wants siren-water.)