If you're using a FF just for a chart plotter, get one that has an internal antenna. Besides a wire for power, all you'll need is a clear view of the sky.
I just did all my electronics myself. I don't like using split screens so I have a 9" Lowrance for a chart plotter and coupled to my auto pilot and a 7" Lowrance used for the FF. Both are touch screens. Added a Fish Hawk X4D for the down riggers. All my fishing is done trolling. Lowrance isn't...
About 7 years ago I was trolling Lake Ontario for salmon on the last day of April. I normally fished alone, as I was retired with all my friends still working. I had just brought one to the boat and threw it in the cooler, and I went to wash my hands in the lake, and just then, for some reason...
You might be able to change the screen on your FF to a flasher. If you get the sensitivity right, you can pick up your jig in the water to put it right on their noses. But even that won't help if they don't like what you're throwing at them.
As long as you don’t have any water in the lower unit you’ll be fine. If it get’s real cold for a night or so, just shine a spotlight under it. Grdbreaker’s tips are good also.