Sending pressure-wave signals underwater could be an interesting communications method, but I don't think any off-the-shelf POTS modems are going to be a good fit for the task. High-speed modems require a bidirectional link with stable characteristics. Any time the characteristics change significantly, the modems will have to spend a few seconds retraining each other. Such changes could occur when the sub approaches obstacles--not a good time to lose communication. Lower-speed modems are less sensitive to transmission characteristics, but noise at certain key frequencies will render them completely unusable; they have zero ability to adapt and select other frequencies.
Digital signal processing is sufficiently cheap that I would suggest rolling your own communications method. I would suggest using something like quadrature phase modulation on many independent frequencies at once with a low data rate at each frequency (maybe 50 baud), and design your communication scheme with enough redundancy so that even if roughly half the frequencies are unavailable you will still pass enough information to be useful (e.g. use 48 frequencies from 10KHz to 18Khz to send 16 bits of data and a 32-bit CRC, and assign the data from each frequency a "confidence" value; if the CRC comes out wrong, find the combination of bits with the lowest confidence values which, if flipped, would make it right).