Pinkie wastes no time jumping into the water, her cannonball making a much bigger splash than it should have. You take off your shirt, shoes, and socks, tossing them behind you one at a time. You then remove your jeans, knowing no one here cares about 'public indecency' since ponies are almost always naked. However, you keep your underwear on. You'd like to keep some sense of decency.
"Hurry up, Nonny!" Pinkie excitedly calls. "The water feels great! It's all wet and stuff!"
"Hold your... Never mind," you sigh. Knowing Pinkie, she would probably take it literally and run off to traumatize some poor, unsuspecting ponies.
Taking a deep breath, you jog a few quick steps to build up momentum before leaping as far as you can, curling into a ball at the apex of your jump and plunging straight into the Pinkie-infested water.
You stay underwater for a few moments, letting your body get acclimated to the sudden temperature change. It's not as cold as you expected, but it still has a slight chill to it. And Pinkie was right, it is indeed wet. Your lungs start to burn after a while, so you finally emerge with only slightly less elegance than an injured walrus.
Catching your breath, you scan the water around you for your partner in pink. You catch sight of her a few dozen feet upstream, now wearing a snorkel she somehow got a hold of. You must have drifted away with the slow current while you were underwater. You start to lazily doggy paddle back upstream, enjoying the combination of warm sun and cool water.
The time passes by in a blur, your mind being occupied by splash fights, a swimming race, and Pinkie nearly swallowing a frog. Twice. Soon, you find yourself exhausted. Either swimming is more difficult than you remember, or you're out of shape.
You drag yourself out of the water and onto the grassy bank. Your limbs are sore and your chest hurts, but you don't mind; you had fun. After getting dressed, you spend the next few minutes watching Pinkie play with various aquatic animals. At one point, she finds a duck and starts imitating it, the both of them quacking noisily back and forth.
Eventually, Pinkie dives underwater, popping up near the water's edge a few moments later. She hops out, now snorkel-less, and starts shaking the water off of her like a dog. Her tail swings around like a windmill, throwing water in every direction. When she finally stops shaking, she appears completely dry. Her mane fwoomps out, becoming large and poofy once more.
Satisfied, she nods once and skips over to you, smiling all the while.
"Quack!" she greets happily.
"Quack," you reply with a small smile.
Pinkie's bubbly attitude slowly morphs into one of serious contemplation. She sits down next to you and places a hoof under her chin, staring out at nothing.
"Huh. I never thought about it like that before," she mutters. "This changes everything."
You have no idea what's going on, so you just plop a hand between Pinkie's ears and start scratching. She closes her eyes and leans into your hand, humming contentedly. She sluggishly lowers herself down until she's sprawled across your lap. A conversation from earlier makes its way back into your thoughts.
"Pinks?"
She twitches an ear to let you know she's listening.
"You mentioned you had a sister earlier. What's she like?" You imagine an entire family of nuclear-powered party ponies on a constant sugar rush. Somehow, you manage to stop yourself from shuddering.
"Hm? Oh, you mean the one I taught how to swim?" Pinkie cracks open an eye to look up at you.
"Yeah, her. What's her name?"
"That's Marble. She's the baby of the bunch," she says fondly. "She's only a few minutes younger than I am, but she'll always be my baby sister to me."
A few minutes? So that means... Oh. Oh no.
"So... she's your twin?" you ask, fearing the answer. One Pinkie Pie is already more than enough. You don't think you could handle another one. Ponyville doesn't have enough yarn for that.
"Fraternal, yeah," Pinkie nods, not noticing your sigh of relief. "Aside from sharing a birthday, we actually don't have that much in common. She's veeery shy, and I'm... well, you know," she says with a sheepish grin. "Limestone is the oldest one. She's very responsible, so she's gonna take over the farm one day." Her eyes suddenly light up. "Then there's my other sister, Maud—she's the second oldest, by the way. She's crazy smart and knows a ton about rocks. She's actually getting her rocktorate in rock science pretty soon!"
Rocktorate? What?
"Me and Maud are super close; we're basically the same pony. Actually, now that I think about it, you and Maud would probably get along great, too! If you want, I could introduce you two the next time she visits Ponyville," Pinkie offers.
You freeze, one line bouncing around in your head: basically the same pony. You rack your brain trying to think of a nicer way to say 'hell fucking no.' Pinkie, noticing the scratches have stopped, sits up and pokes your nose once before flopping back down on your lap, this time on her back.
You blink in surprise. "Did you just...?"
Pinkie pointedly wiggles her hooves in response, an innocent smile on her face.
"Don't give me that look, young lady."
She pulls her ears flat and sticks her lower lip out.
"It's not gonna work this time." You are stronger than before. You can resist.
She slowly blinks her large, baby-blue eyes.
You dig your fingers into the grass, not trusting your hands. You feel a cold sweat break out on your forehead. A silent agreement forms between the two of you, beginning an impromptu staring contest.
This is it. All those hours spent in front of the mirror will finally pay off.
Pinkie sits up in your lap, not breaking eye contact. The seconds drag on, each one more intense than the last. Your heart races, your eyes burn. Nose-to-nose with your greatest adversary, you feel your strength begin to waver. She's smirking now.
In the edge of your darkening vision, you see a small, pink protrusion sticking out of Pinkie's mouth. It slowly extends out before lightly flicking the tip of your nose.
You blink.
"D-did you just lick me?" you ask incredulously. "That's cheating!"
"You didn't make any rules~" she sings, flopping back onto your lap. "I would like a tummy rub as my prize." She bats her eyes at you, gesturing to her stomach.
You take a moment to recover, still dazed by the fact that she licked you. Seriously, who does that?
Suddenly, an evil thought enters your mind. "You're right, I didn't make any rules." You cross your arms and smirk. "I also didn't say anything about a prize."
"Wha- Bu-... Huh?" Pinkie looks like you just told her Santa isn't real.
"Besides, I don't think cheaters really deserve tummy rubs."
The pink mare in your lap twitches a few times, her eyes darting back and forth as she tries to come up with a rebuttal. Eventually, she gives up and sighs. You decide to have mercy; she's suffered enough.
"You're lucky you ponies are so adorable," you chuckle quietly to yourself.
You place a hand on the center of her tum and start lightly stroking in circles. The effect is immediate. Pinkie goes limp, her legs flopping to either side. A silly grin is plastered on her face, the tip of her cheating tongue poking out between her teeth. She starts to hum again in a single, continuous note of bliss.
You've felt Pinkie before, but it still surprises you just how soft she is. You aren't sure if it's a pony thing or a Pinkie thing. Probably a little bit of both. You continue the tummy rub, occasionally switching hands just to spice things up.
At some point, you notice the humming stop. A quick check shows that Pinkie is asleep. Or is she? You poke her stomach once. Her lips curl into a small smile, but she otherwise doesn't react. No giggles, no snorts, no giggle-snorts. She's definitely out.
You huff, knowing you can't let her sleep for very long. It's been about two hours since you left Rarity's, so you'll need to head back there soon. You lean back on both arms, wondering what the best way to wake Pinkie would be.
Before you can think of anything, Pinkie mumbles something in her sleep and curls into a ball. A fluffy, pink ball of cotton candy. She starts to quietly snore.
Well, nobody said you had to wake her up. Rarity can wait a few minutes.