[It happens so fast that even Keith's scream of pain is muffled by the water, which quickly fills into his lungs as he finds himself completely submerged, nearly to the point of blacking out right then and there - the little bit of consciousness that he hangs on to (struck with excruciating pain as it is) serves him for only two thoughts: Self preservation and Shiro. He doesn't know what happened, everything too fast to process, but if he's been attacked, it's just as possible--
Maybe he could have reached for his blade, even almost tries to just on the instinct and panic alone, but the pain and the lack of air is ultimately too much. If he sees the faint shape of Shiro swimming towards him in the water, he might almost wonder if it's his wishful thinking (almost just as much as the thought of No no no, go, be safe hits him at the same time) before Keith just... passes out.]