Why not send the data to satellite only when the aircraft is flying over water?
Remember how many occasions the FDR has been damaged, severely, and taken huge skill and advanced technology to decipher; once it had been found in deep mountain jungle. The data was not 100% available. The same with CVR; often very little recoverable. I would suggest that more a/c crash & burn not over water. True, trawling the depths are expensive and difficult, but high in the mountains is not easy either.
The cost saving to limiting to certain routes would be peanuts.