This was originally sketched out as a more serious fight/sparring match, but then I went to the beach... So now they're having fun and playing in the water. Besides, they look like they need to spend more fun time together.
The thing I was trying to do in the sketch was work on getting ponies to twist and turn. Celestia has a more dramatic turn of her entire body and Luna is a subtle twist of her back end.
edit: Uploaded for two hours and it ends up #2 on EQD Drawfriend [link]
Whoever submits this stuff of mine, I love you.