It was either a water fight or being dragged into the water to swim (which he'll do later, with Quintus' help, probably). Akito shakes the sand from his hair and retaliates in kind, because what good is being on a beach if you don't have fun in the sand, too?

"You're not allowed to sneak off before swimming with Quintus, okay?"

It's phrased as a question but they both know it's anything but.