I posted a fair while ago about the problem of having what seemed like an OAT reading on my MFD's, rather than seawater temp. Going into either of the transducer setup pages told me the actual TD temp, but gave me a message this was not the preferred source. This only happened with the ignition on. I unplugged a Temp-marked  sender up near the control loom, no difference. Raymarine forum told me the TD should be the preferred source. No help.

It was picking up the TD when ignition off, wave a heat gun at it and temp rose. So, today, I decided to investigate further--no manual selection of temp source in menu, but there was one for speed through water, and it was a Merc device. "oh, that would be that paddlewheel sitting coiled up in the well. " Wonder if this has a temp sender in it? Unplugged, turned ignition on, problem gone. And when I crawled under the back to tidy up, I find it is labelled on the loom "paddlewheel/water temp."  

So, if you have mounted the paddlewheel supplied, you would have had no problems. I felt I didn't need it,so it sat in the well, to be dealt with "later".