OEM set-up is for (1) battery, someone added the 2nd battery. The red + cables should go to the battery switch NO BLACK CABLES at the switch, here is how it should be wired.
Both black (-) battery terminals are in parallel, one battery NEG to the other NEG, then the Black NEG cable to the engine block.
Batt 1 red POS cable to battery switch post 1, Batt 2 red POS cable to switch post 2
Main red batt cable to switch post "Common" then to engine starter.
If batteries are good either should crank the engine easily.
Hot cables are either wired wrong and shorting - very dangerous - uncommon
Hot cables caused by overloading (bad starter, seized engine, etc)
Hot cables from internal corrosion (added resistance from corrosion) Replace Cable
Hot cables from poor connections at either end-correct connection
I would not operate this boat until this problem is found and corrected