While on the beach, Fluttershy has to pee. So, as usual, she just does it in her wetsuit. Though Twilight told her people pee their wetsuits all the time, she does get slightly nervous when she does it in front of her friends, as Rainbow Dash and Applejack can clearly hear the sound of her peeing. But for the most part she’s not ashamed, and enjoys the warm release. Of course she’ll have to spend the rest of the beach trip soaking in her pee until she can get home and drain her wetsuit, but she doesn’t mind.
 
I know, just a text inserted in for the sound effect, but really that’s the only way I can depict her peeing her wetsuit because it wouldn’t show. I experimented around with what would look best for that and this was the result I liked most.