He makes this delighted noise at the challenge, something that's not quite a laugh but close enough. It's childishly happy, joy just crawling up out of his chest at the chance to do... well, anything with another person basically. He let's her take the lead across the sand, slowing slightly as he actually touches the water for the first time today. It's cold and he loves it.

Instead of diving once it's deep enough, he dunks his head under the waves and kicks off the sand. It's not as fast starting up, but he stays under for a long time, moving smooth through the water as he follows her. And follows her. And... just doesn't really come up at all actually? There's no air coming from his mouth, but there are little tiny bubbles spilling up around his collar, and he's just going to stay under apparently. He's keeping her in his peripheral though so at least he follows just fine.