RainbowDash takes AppleJack to a nice bay to go swimming the water is calm, the scenery is calm, heck, even RainbowDash is feeling calm. But would you be calm if you mistakenly kissed your best friend under water? NO, see, She has a perfectly good reason to be freaking out! Her best friend probably hates her guts! Or, maybe she wants to make out some more...

(update) I'm leaving the ending it open for interpretation, make your own assumptions