Examples of fluid (including air) being used to transmit digital data?
Mercury delay line memory used sound pulses in mercury to transfer bits. The advantage of this (compared to electrical signals) is the relatively slow propagation speed of 1 450 m/s while electrical signals travel at speeds above 100 000 000 m/s. 

This slow speed was taken advantage of to create memory. An emitter and receiver were connected using a column of mercury. Storing a bit is done by emitting a pulse into the mercury. This pulse will take some time to travel through the mercury to the receiver. Once the pulse reaches the receiver it can be re-emitted again (and again and again...), allowing the  bit to be stored until no longer needed. More data can be stored in a single delay line by sending out trains of pulses.

UNIVAC I is a famous example of a computer using this type of memory. It stored 120 bits of data per column.