Rolf knew his limits, and he wasn't in danger just yet, though he was getting close to needing to surface. However, his arm wrapped around her waist and his surprise had his mouth opening under hers, allowing the exchange of air. He kicked automatically as well, a little overwhelmed by the sensory input he was getting.
He had daydreamed of kissing her, both in the far past and most recently. He certainly hadn't expected it though, even in this manner. They broke the surface and they both gasped for air. One hand found its way into her wet hair; the other helped to keep them afloat by treading water. "Rosie," he whispered, his forehead tilting to rest against hers. His lips were a breath from hers, then ghosted over hers lightly, salty from the sea water but warm. He didn't know if she might want it or if he'd mistaken her help of air for something more, but he couldn't seem to help himself.