After much searching and trial and error. I have finally found the solution for my HDS 7 Touch reading water temp from the epoxied transducer in the bottom of the boat.
Change your 83/200k sonar transducer source to one that doesn't have Temp. for those freqs. put in PD-WBL
This will force unit to get the water temp from the outboard structure transducer.
Have you confirmed that this solution works? Others have mentioned trying this without much success. What software version is your touch unit currently running?