Castiel looks over when the Doctor yells, too late to do anything but realize that they've just lost someone and he hardly has the power to revive anyone here.

He grips Donna tightly. "We should get back to shore before it makes another grab for one of us."

Part of him wants to go after Wallie--feels that he should, but if he does and it gets Donna or the Doctor killed, that would be worse. He needs to keep them both safe.