I see that Lowrance bought out one of the "castable" ball-shaped fish-finders. It will compete with Deeper, another popular model of the same sort.
I know they are limited, but for someone looking for general information: contours, structure/cover, depth and water temp, I am surprised these aren't way more popular for kayak anglers. Maybe they are and I just don't know it.
On the Lowrance model, Deeper, too, you can drag the thing or cast it and map out a lake's contour.
Anyway, if nothing else, it does away with all of the mounting issues.