Other mistake: When Truman is standing at the open doorway at the very end, and is addressed directly by Cristof, during some of the shots, the open doorway behind him becomes a black backing board, visible from the way the light falls on it. (01:32:15)
Other mistake: After Truman opens the door to the outside world there is a close up shot of him listening to the creator. Behind Truman is a solid black wall, not the shadowy space we would expect and do find in later scenes.
Answer: He'd been hoping the waves would discourage Truman and make him turn back. It's a lot easier to agitate water than it is to calm it, so by the time he saw it wasn't working, even if he's thought of that, it would have been too late.
Captain Defenestrator