IMHO, if everything is working the way it's supposed to be working, I'd just ignore it. There are always extra wires running everywhere on a used boat. Impossible to know what they were used for without a manual from the maker. As to that large wire, could be for an anchor windless, or a trolling motor or anything else the requires a larger gauge. Might even have been for a spare battery. 
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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