(Assuming that the character was male prior to the change)
You step down into the water and near instantly you could feel the change starting. It's a strange sensation running from your feet up through the rest of your body that is a mix of both pleasurable and painful at the same time as your features change and your bones are modified.
You are forced to sit down and any clothing on your body prior to the change would promptly be discarded, so that it wouldn't get in the way of your transformation.
Your face became more feminine: your eyebrows thinner, your nose smaller, your lips fuller, your ears became pointed and a bit fin-like as if a precursor of things to come.
Your arms, hands, and fingers became more slim and your fingernails grew to be a bit sharp but not overly so.
On your chest, two mounds raised up and balloon out until they are at least DD in cup size. Your waist became thinner in response as if the change was simply shifting your preexisting weight around, though this didn't seem to be entirely the case as your hips similarly grew in their size to give you some nice curves.
By this time, your package had shrunk down to nothing and had been replaced by a vagina, making you a bit uncomfortable at first at the void there now seemed to be between your now feminine legs.
However, you wouldn't have those legs for long as the skin cells on them grew, fused together, and became hard until they were each the size of your thumbnail. These were your new scales and had a pinkish-purple look to them. The skin at your waist seemed to fade into your scales instead of cutting off abruptly.
You lost strength in your legs as they were merged together after your legs had been completely covered in scales. Your bone structure was changed to become more like a fish from the waist down, so that your tail was more flexible.
The scales covered your new vagina as well as your backside, which was shifted to be under your private parts to be more in line with aquatic mammals. It gave the appearance that there was no opening on your new tail but you could sense that they were still there. Your behind lost its definition in the process and became more of a single mound.
Finally to finish off the transformation, the fins and other "additions" sprouted on your changed body, all a mix of pink and purple like your scales were.
A large two-feet-long three-feet-wide fan-like caudal fin sprouted from the end of your tail while a smaller fin grew on each side of your unified shin. On top of your single thigh, two more fins sprouted on each side of where you could feel your private parts now were. A flexible fish-like dorsal fin grew from your lower back where your skin met scale. Finally, one more fin grew on each of your arms from your wrist to your elbow.
On the sides of your torso, under your new breasts, grew two sets of three gills that opened and closed softly. Strangely, even though they were above the surface of the water, you didn't feel like you were choking.
You release a soft sigh of relief as the transformation finally stopped. You smile as you look down at yourself, admiring your new body as the scales shimmered and your skin had that wet, slick, glistening look that some girls on the beach had.
You were interrupted by a sigh of relief coming from the mermaid who had been in the pool before. Curious about what brought that on, your gaze shifts to that of your new mermaid friend.
It seems she had lied to you, or at least partially. While you had undergone a transformation into a mermaid, she had changed back into a nude woman. She wiggled her toes while her eyes were closed as if it had been forever and a day since she had been in such a form.
"That feels better...," She said as she gave another contented sigh in relief.
She had lied to you about the transformation, could she have lied to you about the tunnel leading out of here?