I don't think you will be happy with the portable Hummingbird strapped to the yak. A more secure mount like PVC in the Sail Mast, or a $25 Ram Mount would serve you better (lots of maneuverability options).
-If its not having wires drilled trough the hull that you are after consider mounting a PiranhaMax 215 ($89) this way:
On the Adventure there is a T-Cleat in the forward section of the well. If you remove the T-Cleat you can drill a small hole behind it that will allow you to pass the FF wires through. When you don't want wires showing through your Yak, just slide them back into the hole and re-install the T-cleat.
-This unit runs for several outings on 8 AA rechargeable batteries that I have in a $6 dry box from Walmart, that is Velcro to the inside of the hull.
Additionally, I always use a 1 qt. freezer bag over my FF and use a rubber band to hold it on. It completely covers the unit and wires. I have taken big waves to the chest while launching and the FF remained dry and still works perfectly. (Still get the West Marine Warranty though)
