so imagine this

Cadance and Flurry get foalnapped by Chrysalis
you mentioned watersports, so i guess this might work for you
she has Cadance tied over Flurry

"Oh, look at your little one, so blissfully innocent and ignorant of the situation. Tell me, princess... how would you like me to be the first to show her pleasure?"
of course, Cadance would protest and refuse

"Oh, you're not happy about it at all? not even a little? Are you suuuuureee?" And Chrysalis would start to tickle her, using her magic to torture her with it. "There we are, much better. Look Flurry, mommy seems soooo happy to see you prepped for me. Cootchy coo, princess, you like this, don't you?"

Cadance would laugh against her will, and start begging to be taken instead.

"Oh, i'm afraid your husband has already had you, hasn't he? But we can make a game out of this. I'm going to tickle you for say...oh, an hour.  if you can last? Then I guess I'll let my children have their way with you as I watch. But, if you lose it, and piss all over your daughter....Well, then I'll just have to have her washed  in such a delicate way, won't I?"

 Cadance is forced to laugh and giggle, she does her best to tell her daughter it'd be ok. but such a naive tot sees her mommy laughing and just claps along as Cadance does her best to hold it in. Chrysalis just laughs "Awww, well look at that. Even your daughter knows you're having such a good time. Tickle Tickle little princess, don't go having a tinkle tinkle now"
Cadance would be having a genuine cry after some time, her tears seeming like tears of mirth as the merciless tickling brings her to slight coughing fits within her laughter
it wouldn't be long until Cadance loses control, and pisses her yellow liquid all over Flurry like a warm shower. "Uh oh, looks like mommy couldn't hold it. Children, be a dear and get me some soap and a towel...and make sure to gag Princess Cadance, I don't want her spoiling this next part"

Flurry, so innocent and dumb, would just clap her hooves ignorantly as her mother's giggling dies down, her own mane soaked with piss

With Cadance gagged, Chrysalis would take the time to comfortably tie the foal's legs, spread out, on her back. She herself would be disguised as her mother to keep the baby pony happy. "Oh, my poor little filly. I didn't mean to piss on you....do you want mommy to clean you up?" She'd say while evilly looking up at the real Cadance

she'd use the towel to gently start cleaning up Flurry, licking her muzzle as she very delicately rubs it across her belly to make her giggle and sputter. "So innocent...so naive..." Cadance would fight and try to scream, but all she could do was watch as Chrysalis's soothing clean up soon turned to tickle torture on the foal's tummy. Chrysalis just cooed as the baby became uncomfortable, laughing and struggling, small strains of groans as the baby was forced to laugh for her amusement. "Watch this, Cadance..." She said as Cadance herself cried as her baby was obviously tortured with laughter, now even knowing how to say the word "Stop"

"Pee for mommy~" Chrysalis would say as she gave the same tortourous treatment she gave Cadance, uncaring of the young age of the foal

Flurry, would soon piss her diaper. Chrysalis, taking her 'mommy' duties seriously...would slowly take it off and toss it aside, eyeing the shining slit of the little filly "Oh my, it's sooooo cute." She'd take the towel, and give it a few pokes, which would cause the little foal to giggle and sputter. "Oh, and quite ticklishly sensitive too! adorable!"

Flurry would just look at Chrysalis with a blushy shy look, unable to comprehend what was going on. While her mother,tied above, would be yelling into her gag with furious anger
Chrysalis would just continue to dab the towel onto her little pussy, sometimes giving a slight push into it, and sometimes wiggling it on her tiny pussy rim to make the foal squeal. Everytime she gave it a dab in the right spot, Flurry's tortured giggling would give way to a babyish coo or moan.
"You're simply enjoying it, aren't you sweetie? You simply love it when your mother cootchy coos your little sexy slit, don't you?"

Chrysalis would say with a chuckle

Chrysalis would keep up the pleasuring and slightly tickle torture of the young foal until she was noticeably equal between her giggles and moans. Then she'd look upon her with a disgusted look "Unfortunatly for you, I didn't bring you both here to enjoy yourseves." She'd say as she magically raises feathers above  Flurry. "Tell me Cadance, what do you think will happen first? Will she have her first orgasm, or die laughing?" Chrysalis would then spread the tiny pussy lips of the little tot, and use the feathers to brush along the sides of her labia, she'd then produce a small brush and a long thick feather, one feather to stroke across her tummy while the brush dabs and gently brushes at her little foal clit
Flurry would start shaking,bouncing, and screaming in uncontrollable gurgles and giggles as her body is tortured ticklishly and sexually
unable to even say a word of protest

Cadance was crying, defeated, as she watched her child be tortured at the hooves of this monster. Flurry, however, being an alicorn, wasn't giving out anytime soon. but any relief the little child would have from an orgasm, which was obvious she was having whenever her body shuddered, was drowned out by the pure peals of laughter she was squeaking out

parts of the young foal covered in her little filly cum as she bounced and laughed and nearly choked. Even Chrysalis found it impressive she lasted this long. Even further, she found Flurry's smiles and giggles not only to be cute...but filling. After Flurry's sixth orgasm, she decides to stop. Flurry just laid there, breathing hard, crying, unable to move. "Oh...you will make quite the cute little tickle slave, little one. As for you, Cadance...Have fun with my children, they'll sure to be a laugh or two"