I haven't reviewed your installation in a while but I believe you are running 4 12v batteries in series. Each 12v battery consists of 6 2v nominal cells that when charged are a touch over 2.1v.
Likelyhood is that one cell has had crumbling lead antimony plates that may or may not have shorted together in the bottom of the battery, resulting in low charge capacity or effectively no capacity.
The other cells (not just in the one battery but in the entire string) are driving current through the bad cell. When the bad cell's limited capacity is exhausted, the rest of the battery will drive the cell into reverse (Think of it as charging the cell backwards. at 46 volts). This further damages the plates in the cell, causes electrolysis and offgassing and generates heat. The cell, instead of being a producer in the circuit, has now become a consumer/resistor. Further more, once the cell is in full short, when you charge your whole bank you wind up OVER charging the rest of the cells to make up the bad cell's lack. You're in a better position than most LA/SLA users with individual battery voltage monitoring, Mike, so at least you can catch this.
Will it catch on fire and burn your boat down? EHHhhhh... depends on the exact nature (impedance of the short), the current being driven through it and time, probably not in the very short term. But at the high currents used in electric propulsion it would scare the crap out of me.
<-- disclaimer, BS is going to say that i'm giving you bad advice and am trying to kill you with that statement. I'm just saying what you saw; you were able to continue under gen assisted motoring and get back to harbor without a fire. But I'd be as uncomfortable with the situation as you clearly are so, all good.