As far as I'm concerned, the Humminbird thru-hull is THE way to go. They'll exchange the transducer that comes with a new unit for one of these, no charge. The unit protrudes from the hull bottom less than a quarter-inch and is smooth and flat, so there's no extra drag or snag worries. The only drawback is that there's no temp sensor in this xducer.
Here's the unit from the outside:
And here's the inside:
Sport Cases (Electronics)
Cartop Kayak Carriers