SparkFun Forums 

Where electronics enthusiasts find answers.

All things pertaining to wireless and RF links
By itolond
It’s to control via 2 way coms to an app for a model submarine
Depth max 5m
Water varies, but worse case salt and Low visibility
Distance max 300 m
By jremington
Sadly, underwater transducers are expensive and for 300 meters range, generally require high voltage electronics for the transmitter.

You could experiment with a simple piezo disk as a transducer, but they are not efficient at transmitting sound energy into water.