I have heard that cathodic protection will only be effective if the metal is submerged in water and is ineffective in dry conditions?
That is true for systems that purely use one technology. Impressed Current on its own is effective in dry conditions but cannot impress enough current to be effective in the wet; Sacrificial Anodes using cathodic protection is very effective in wet conditions but far less effective in the dry. RustStop is the only system available that combines both of these to get the best of both technologies.