I dont think that’s a clearance issue with your livewell screen, I have the same setup as yours and mine works just fine. Actually, my livewell screen sticks out even further than yours does. The only other thing that I can think of is if there is a lot of turbulence from rivets, welds… being created by your hull at the very point that you’ve installed the transducer.
This really sucks for you, the screen shot you posted is really crappy. With 17 ft of water and 3mph you should be getting excellent images.
Good luck!
EDIT: Oh yeah, one other thing. There’s two different frequencies that you can run on your SI, try both of them. I get almost nothing with one and perfect images with the other. (I think I use 455khz all the time, and the 800khz doesnt work no matter what I do, or something like that, I dont remember the exact frequencies sitting here at the computer.)