Hydrophone kit to transmit (and receive) underwater
Yes, may be you are confused between Hydrophone and Modem.Hydrophone is just a passive device that records sound underwater. And what you just describe in your question is modem, which emit and receive sound waves underwater. there are definitely so many options you have. There are number of factors you need to consider before buying, mostly the operating frequency as most of the equipment in underwater works with KHz frequency. There are some amazing Software Defined Modems are available in the market but definitely those are expensive. If you want to just design and develop some communication protocols then you can use the UnetStack, basically convert your laptop sound card into a modem and have pretty good functionality. If you’re planning to buy some cheaper uw-modem, you can try buying popoto development kit, which includes transmitter and receiver. If you’re interested in some of the hydrophones then it worth give it a shot to Oceansonic icListen SC2, i must say this is an impressive military grade performance Hydrophone. This guys also had icTalker, but this can not generate custom signals, but definitely generate some chirp, continues sweeps, tones etc.
Also there are good market players in underwater domain such as,

Subnero
Benthos
EvoLogics GmbH
WHOI Micromodem
Popoto