transmit data wirelessly under water?
Device is called ultrasonic transducer and it's freely available on a market.

For communications you will need MFSK modem 

http://acomms.whoi.edu/umodem/

http://cseweb.ucsd.edu/~kastner/papers/oceans10-low_cost_modem.pdf 

or write your own protocol and software.

Note that communication will be short range, if it's ROV and you don't consider it to be AUV it's better to use simple Ethernet cable. 

Military submarines also use for underwater communications ELF, VLF radiowaves, so you might also consider this option for AUV, but data rate is very low on this type of systems.