He knows when she goes down that she's going to move, because that's the right thing to do in a battle like this: you don't just camp in one spot and wait for the enemy to beat you down. So he does the same thing, as they both drop swiftly underwater, hidden from each other by the movement of the river, the dark shadows, the glittering moonlight.

Swimming several yards upstream, he carefully resurfaces, and... shit, he can't tell where she ended up. If he whirls around too quickly it will almost certainly give him away, but. This is life-or-death here, he can't just let her get him--

Oh. There she is: he just glimpses the shine on the black streak of her hair. Omega glances toward the handful of mud in his hand underwater, and then drops it into the current. Then he promptly sinks underwater again, and swims downriver toward her -- maybe he won't grab her by the legs, but he can definitely take them out from under her.