Need advice on installing FF to tandem kayak?

[YakDout]

And for the first couple questions,

To get full functionality of your hdi transducer, it should be in the water. Ive seen guys use sprinkler parts to go through a scupper hole and recess the puck flush with the bottom of the hull. Some kayaks are easier than others depending on shape. If I had a choice of mounts for the FF I would go with the short gear track rail and ram ball on gunwale. It will look cleaner and be out of your way.

If you do a search on here, some things should come up for the sprinkler rise transducer mounting.