I know its extremely difficult and a lot care is required. Every environment is different. Coral rubble in the waves, boats passing by, depth, salinity, etc all play a huge roll.

If you describe what you are working on and what you need to do maybe a solution can be found? 

It's probably much easier to find a company selling acoustic modems than to try and create your own...very challenging environment to work within. I have about 10 years working with underwater acoustics (as a non-engineer). 

To give you the best advice, I need a bit more information about your specific application. Underwater communication typically relies on acoustic modems because radio waves don't travel well through water. For longer distances, a hybrid solution is often used. This could involve an acoustic modem transmitting data from your underwater sensor to a surface buoy or platform. The buoy could then relay the data to shore using traditional radio communication, such as 900MHz radios (even commercially available options like Ubiquiti radios, depending on distance and line-of-sight). To help me understand your needs and recommend the right approach, could you tell me more about:

Data Rate Requirements: How much data do you need to transmit, and how often?

Range: What is the vertical distance between your sensor and the surface receiver?

Environment: Are you working in shallow or deep water? What is the water temperature, salinity, and expected noise level?

Power Budget: How much power is available for the underwater sensor and modem?

Cost: What is your budget for the acoustic modem system?

Integration: How will you integrate the modem with your sensor and surface receiving system?

Real-time or Delayed: Do you need the data in real-time, or can it be stored and transmitted later?