Your eyes are drawn to the bright blue Speedo-branded swimming jammer, so you grab it with a matching swimming cap and goggles, even though it's too big for you.
In the large changing room, there is a mirror taking up the whole wall. Eager to try on the jammers, you take off you clothes until you are completely naked in front of the mirror. You pull the jammers up your legs, shivering with pleasure when the soft material strokes your penis. You also put on the cap and goggles, thinking to youself that you would look like a professional swimmer if you had a bit more muscle. As soon as that thought enters your head, you feel a weird sensation for an instant, almost like you have pins and needles in your whole body. However, the feeling remains in your feet, and you gasp when you see your average-sized feet stretch and widen to a size 14, perfect for propelling yourself through the water. The tingling feeling then moves up your body to your legs. You wince as your calves inflate and tighten until they become the size of American footballs, and your thighs swell until they stretch the material of the jammers. You moan as the tingling reaches your groin, and you can't help but rub your growing cock through the swimsuit. You stroke faster as it thickens and grows to 10 inches long, clearly visible at all times through your jammers. At the same time, a perfect bubble butt pushes out the back. Eventually you climax, splattering cum all over your shifting chest, landing in the crevices of your new tight six-pack. Your previously non-existent pecs bulge out and flatten into rock hard armour-like slabs of muscle, making your expanded nipples stand to attention. The tingling spreads up your neck, stretching your spine and bringing you up to an impressive 6'3, and your shoulders, which crack and widen before swelling with your taut back muscles, toned from years of swimming backstroke. You flex your ballooned biceps and rub your new vein-filled muscular forearms with your lengthening meaty hands. Your chin cracks into a masculine square shape as your light stubble grows out into a short beard. Your nose snaps into a straight shape as your nostrils fill with the smell of chlorine. Your neck-length black hair lightens to a light brown and shortens into your swimming cap. The tingling feeling disappears and you feel dizzy for a few seconds. As you regain your composure, you stare at your new body in the mirror before being interrupted by the shop assistant from earlier.
"Ah, I see you're done! Follow me to the pool."
You follow him through the back of the shop as your memories are rewritten, to being the best swimmer in your swim club, to entering national competitions at a young age, to entering the Olympics at 19. You shake your head and stretch your arms, ready for an intense workout in the pool. The shop assistant opens a door to a large Olympic-size pool, stepping out of the way.
"Enjoy."
On his way out, he 'accidentally' brushes his hand against your obscene bulge, winking when you turn around. Smiling to yourself, you stroke your cock to get yourself hard and wrap your toes around the pool edge. You lean back and dive in, starting the swim of your life.