Could cause BOTH cables to heat up if they are same diameter conductor and same condition I would expect? but often they would not be any hotter than if everything is ok.
Consider most circuits as Circles hence the word.
Current flows from pos to neg or not at all.(BTW Actual electrons supposedly go the other way) If there heat or work a circuit is complete, Even if it is Shorted somewhere along the way.
But your description sounded like starter gear was not engaging flywheel even though starter was turning.
pinion isnt moving into flywheel. If cables are clean and tight
most common cause is Corosion/lack of lube right close in there.
does your starter motor look like this ?
http://www.marinepartsplus.com/catalog/category/yamaha/Outboard/1989/115ETLF/620304280017/