An intensely strange sensation passes through you, and you feel it seem to settle in your chest and crotch. Before your eyes, your nipples seem to swell up and feel sensitive. You move your hands to them, faintly noticing they have shrunk after touching the water, and when you rub your nipple you pant in an oddly high-pitched tone. Your chest starts to expand under them, and you see large mounds of flesh form. These are... you're growing breasts!
You blush and your hands move to cover them, and then as they cup the tender flesh you grow aroused at how having them touched feels. Instinctually, you paw at them and moan softly, imagining having them sucked on, until a familiar twinge flows through your crotch and you orgasm. Peering past your new mammaries, you see yourself shoot several loads into the spring, your thick semen soon dissipating and the water growing clear again.
The consequence of your ejaculation soon emerges, and you feel a clenching sensation in your crotch. Your penis slowly shrinks from a phallus to a tiny sensitive nub, which you realise is your clitoris. Under it your testicles tug inside your body and reshape into ovaries, while your scrotum tucks inwards and the seam down its middle forms into a vagina.
You're quite freaked by this change. "Master," you murmur in your now high-pitched voice, "I... I'm a woman now!"
"Of course, Shihomi," he says kindly. You know instinctively that's your name now.