My fishing buddy had a setup on his boat. He wound up using his garden tractor battery for his fish finder because there was too much interference and he had a filter on his power cable to the fish finder.
If weight is a keeping you from putting the fish finder on a separate battery, have you considered a 12volt 7amp battery for the fish finder.
It wasn't just the fish finder - I've got a bilge pump, running lights, a horn and FINALLY a livewell - but yes - space was definitely an issue. I haven't had any issues with interference, running it as depicted in the diagram. Still can't find the card reader to upload the pics, but once I get done with everything I'll get cleaned up in my workroom and I should find it and be able to post them up.
Sorry for the delayed response - yes it did. Though I test the 12v system every time i rig everything up by sounding the horn before I connect the fish finder...if i'm going to fry something let it be the wally-world cheapo horn and not my $600 headunit...
I was able to conceal both batteries in the middle bench - i've added so much foam elsewhere the removal of some in the middle made no difference to me. I used a tempress extra large hatch to clean it up on top. Right now the boat is on saw horses upside down in the backyard burried in snow - I'll get pics of the final product come spring.
HA - must be nice...i just dug out around the bass boat I'm selling so people responding to the craigslist posting can actually get to it...I had just been sweeping the snow off of the cover and letting it pile up around the trailer...dumb idea...
we've got a solid 2' pack of icy snow everywhere, the result of probably 5' worth of snow that's fallen and compressed over the course of the winter. If we make it through spring without a horrific flood it will be a miracle.