If there is any doubt about the procedures, a dive supervisor must make absolute provisions to protect the divers. This may mean pulling fuses out of hoist or pump controls, dismantling valve handles, or even placing a person to guard the point so it cannot be turned on or activated inadvertently. Next, always presume that all openings are not completely closed. Since a great many bulkhead, gates, weirs, etc., are controlled by hoists using lifting cables, a good seal cannot be presumed simply because the openings were closed on the surface. Wire cable cannot force a gate closed or a bulkhead down.
Gravity is relied upon for that.
It is amazing how little interference it takes to compromise a seal. Even hydraulically controlled gates should be suspect unless there is a positive way to measure closure. Most of the time a gate's failure to seal is caused by a rock or other debris on the sill. Divers should always make it a practice to approach all openings with extreme caution at first. If at all possible, visually check the downstream discharge of any valve or gate.