For underwater communication, we need a radio module that we can easily use to send data over e.g. serial port (hardware or software) using ArduinoIDE and an Atmel-Processor.
We have following application: transmit data underwater at least 5m
We thought about following solution: make the frequency as low as possible, we are aware of several radios with 315, 433 and 868MHz but that's still too much for us.
Please, before we give you the job, make a suggestion what radio you would use to accomplish this so we can filter the not so useful applications.