I bought a pontoon with transom mount hummingbird 150 on it , then was given a hummingbird 788 CIhd with no transducer so I just replaced the head unit on the consol and never bought a new transducer for the 788 everything seems to work correctly with the exception of the water temp option which I now see that that is built into the transducer . was wondering if it is worth the time and effort to run a new and correct ducer cable if all I am going to pick up is the temperature option or will new cable have other hidden benefits ?
keep your lines tight as you never know
All you would be getting extra would be the surface water temp. read out like you said the 2D would be the same in both transducers.
Be safe and good luck fishing