Mount it on the jackplate. It'll be out of the water on plane, but it'll give you the best reading when you're idling around looking at structure. And mostly protected from floating debris. Get a Y cable (
http://amzn.to/1w1u3dr) and a second transducer to mount in the hull. This transducer only needs to be standard sonar ducer, so it'll be considerably cheaper than the side imaging transducer. Doesn't need to have a temp sensor either. This is the one I used -
http://amzn.to/1FMwUZ1.
You can also do the same thing with a toggle switch (
http://amzn.to/11RCUAz). I have done it both ways, and there's pros and cons to each. The Y-cable uses the jp mounted ducer for side imaging and the one in the boat for straight down sonar. You give up a little sensitivity shooting through the hull on the straight down view for simplicity. With the switch, you'll get all of your views through the JP mounted ducer when your idling, then flip the switch when you take off to read depth from the in-hull mounted ducer.