yenitngyenta:
1) He can knock on the door. He cannot if the water is going to ruin the entire room outside; once the plug is out he can bang on the door so that the guys will hear him.
2) The servants will come in after some time like they usually do, especially if they don’t hear anything from him. Not if he drowns, they won’t.
3) Thirdly, even if he cannot get out – at least it answers the question, unlike the horses one where you need to solve the problem without answering the question!!