(MC POV)
Tsuki straddled me to deepen our kiss, her arms wrapping themselves behind my neck while her tongue pushed itself inside my mouth.
My hands reached down to hold my little sister's waist, my fingers running over her soft skin under the water which caused her to moan inside my mouth.
Her hips rocked slowly against mine, her entrance rubbing itself against my manhood to full erection.
Tsuki broke our kiss to breathe, her breaths coming out in large gasps. Even then, her hips did not stop moving.
"Aniue… Over here… It's aching…" She whispered, her hands moving to rub her navel.
I smiled at her and lifted her up from the water, my little sister immediately wrapping her arms around me the moment I did so.
I only realised a bit later that we could have simply had sex while submerged in the water since I could simply stop any infection from happening with a snap. Oh well, old habits die hard I guess… And since we were already out…
Just in case you don't know, underwater sex is actually really bad, you get a higher risk of infection and other nasty stuff.