You decide not to wait any longer. Slipping free from Amy and Jessica, you dive head-first into the pool. You quickly realize it's a saltwater pool instead of a chlorine one. A moment after you hit the water the tail costume becomes so tight you can't feel your legs. If it didn't feel so good to be underwater this might cause you to panic. You try to kick with your legs and after a few awkward jerks the feeling begins to return, except something feels different. You kick again and now your tailfin starts to propel you forward. You build up speed across the bottom of the pool then make a 180 just before the wall. Getting the hang of your tail you move faster back the other way. Soon you're swimming with a grace and speed that would make an Olympian jealous.
It takes several laps before you realize you haven't need to come up for air! This time instead of an underwater turn, you break the surface and execute a backflip that clears the surface of the pool. You surface, treading water with ease and taking a breather from your impromptu performance. You realize most of the partygoers are standing around the pool looking at you. Suddenly they break into applause. You smile, wave, and swim over the stairs.
"That was amazing!" says Jessica. "I didn't know you could swim like that. And with a costume no less!"
As the adrenaline starts to wear off the reality of the situation starts to set. You reach down to your waist and feel for the edge of the latex costume. There isn't one. You're a real mermaid now.