An alternate version of this, by request. (Difference is, no pee running down her legs)
 
Rainbow Dash had to pee really badly after drinking a lot to stay hydrated playing volleyball on the beach. She rushed to the bathroom as fast as her athletic legs would carry her, but she didn’t make it. She returned in wet swim shorts, much to Pinkie’s amusement. Rainbow Dash gave her an annoyed look and said that swimsuits are meant to get wet anyway.