As you go further into the water, the tingling increases. Soon, it's like a million pins and needles are being stuck in your skin everywhere the water touches.
Looking down in confusion, you see something strange about your legs. It might just be the way the water bends the light, but they seem to be getting smaller somehow. And then you notice something else...
A lump of some sort is growing from the end of your spine. Shortly, it develops into a fish-tail much like those the mermaids have. Meanwhile, your legs keep shrinking until they're completely gone and you fall forward. The only thing holding you up was your new tail and that wasn't doing much good.
Sputtering, you break the surface to see all the mermaids around you smiling mysteriously.