80% of the time a melted battery post indicates a bad starter or a loose connection. If the battery terminal is loose a spark or short will generate heat and will degrade the terminal. If the battery post was tight and melted that is a sure sign of a dead short in the system. Most times the short is either the starter relay or the starter. Yamaha watercraft are known to show a 12v low while seizing and or a bind in the driveline and this can be another sign of ensuing repairs.