Dammit. He'd told her to hold on, and Laura had, until they hit the water and sank. Human self-preservation had kicked in at that point-for her at any rate. To him, water was just a medium like air or the void of space. In her case, though, she didn't have the ability to survive long underwater, and in her attempt to save herself, she flailed away from him.
Given his hands were busy as he shucked his soggy wings, more weight than he could easily handle right now given the drag, he couldn't spare a hand to clamp her to him.
As soon as he'd shed as much as he could, though, his first priority was to reclaim her, if he could find her.
Despite the protective film the nanos dropped over his eyes, he couldn't see much. Given his orbs were still mostly organic, he couldn't illuminate his field of vision. The murk blinded and he found his movements hampered by the current. He couldn't spot Laura.