Mike,
I have the Cutwater 28 and the 5212 GPS, but I am experiencing the same issue, among other issues. My research my help you and others on these issues. Here's a few things to consider:
1. As the earlier post indicates, the transducer is mounted to the hull. The transducer may be directly attached to the hull using epoxy (the common method used by ranger/cutwater) but not recommended by the transducer manufacturer. Mine is now attached using the well filled with Propylene glycol. If yours is attached using this method, then you may need to fill the well as it does evaporate or sneak out over time...
2. If the GPS unit runs hot, it will exhibit a number of strange characteristics, one of which is no read on the transducer. Others include freezing, random panning mode, unresponsive. If you have a fan, turn on to cool down. Notably, if this is the issue, it is more likely that your problems will start after about 30 mins of operation.
3. I have an SD card with extra maps, and I noticed that this will also cause problems, probably due to using memory. I remove the card when it acts up too much.
4. I noticed that garmin units are picky about memory use, so if you have lots of tracks and routes saved, this extra memory use with also cause the system to do strange things. Sometimes a "factory reset" helps clear this all out (after you save routes and user data to SD card)
5. You may have a defective transducer or GPS unit. My GPS unit has been sent back to Garmin once and now a second time due to these and related technical issues. My transducer was replaced by dealer/cutwater, as it popped off in the first months of ownership (this is the reason why the manufacturer of the transducer does not recommend epoxy, but for some it works).
That's my exhaustive research around Garmin GPS, which as you can see has wasted a lot of my time being on the water!