It is quite amazing how something as simple as a dust cap can be so critically important in the grand scheme of things. One would believe that if you merely lost your dust cap you could leave it be as it is not that important. Well, that could not be farther from the truth and the issue lies with the fact that without the dust cup installed, water can be allowed to seep through. The first stage body can have its internal components exposed to water if it is not connected to the tank and the rubber cap is used to seal the opening.
To keep this short and to the point, do not go out diving if you notice your dust cap is missing.