Could maybe try a small amount of a good wax on a soft cloth to gently rub into the stains and buff out? The wax could act as a gentle solvent and remove the stains. I have seen that on different vehicles over the years with the very hard water that leaves those marks, and have the wax able to somewhat remove them.
I have no idea if this works, but at $12 it may be worth a try if nothing else works. Alcohol has worked for me in the past. For the last few years, I have wiped my screen dry with a microfiber after every wash to prevent this. More often then not, my ocd takes over and the entire boat gets wiped dry though
After washing, both truck and boat get complete wipedown with a good quality chamois. Before the wash, also like to run salt away stuff thru the engine and spray down the entire trailer with salt away too.
Saturate a p towel w white vinegar( 10% acid) lay on screen tuck corners Let sit till dry , wipe w alcohol after. Should look like new. I do same w windshield good luck.
I try not to scrub. Scared of micro scratches
I've never used a magic eraser on a GPS screen but I use it a lot on my boat to get ride of black marks and things like that. I use it to get hard water stains on of my motor and polished aluminum. That's crazy it would scratch the screen. Thanks for the heads up. Those Simrad screens look so clear I'm sure you need to be careful what you rub against them.