A gas meter (100) and like metering systems that are configured for use underwater. The gas meter may embody a positive displacement rotary gas meter having a meter body with impellers that counter-rotate in response to material flow. The gas meter can also comprise an index unit having an electronics assembly to generate a value that equates rotation of impellers with a parameter of the material flow. The gas meter can further comprise a connective interface having a first part and a second part, one each disposed on the meter unit and the index unit, respectively, the first part and the second part coupling with one another to provide data to the electronics assembly, the data corresponding with operating conditions on the meter unit and rotation of the impellers. In one example, the first part and the second part are configured so that the meter unit and the index unit are operable underwater to generate the value.