It all depends on how you crack the hydrogen and oxygen out of the salt water.
Potentially you could power the radio systems from a wind or (more likely) a wave farm. In theory this would be "free" energy.
One question I have is, if you are "burning" hydrogen (at which point it produces water) you are locking away the oxygen. Plants renew our O2 supplies by cracking CO2 and holding the carbon and releasing the oxygen, which they (as far as my GSCE Biology goes) don't do with water.