I do have a battery charger on the boat, it's a 4 bank 40 amp charger. It is charging my cranking battery because I am able to use the battery to start the boat after I have plugged in the boat.HEY!! Do you have a charger on the engine battery? Some boats only have a charger on the trolling batteries (like mine). If you do have a charger on the motor battery, maybe it's no good.
With a 4 (or more) batteries maybe all the legs of the charger are busy on the TM batteries.
It's starting to sound like one of the 36 volt batteries doubles as the starting battery. Since it's been "run into the ground" repeatedly, he might as well go shopping for an AGM size 31 for that slot.
2017 Nitro Z-21 Pro Performance w/ 250HP OptiMax Motor
Minn Kota Ultrex 112lb/36v/45"
2018 GMC Sierra 2500 HD SLT - Diesel