Interesting that it is ONLY between the ESC and the motor where the solder plugs have melted. It is the same ESC, same motor on BOTH boats (one surface drive and the other submerged).
The output side of the ESC and the motor are 14AWG wires and the input side from the battery are 12 AWG wires.
Can only be down to the higher C discharge rating of the bigger battery
Not quite. In a situation with a nice newly made connection, I would suspect the new kid on the block, the connection, as being the guilty party for generating the unwanted temperature in the place where the solder melted.
If a very neat connection between bullet and wire end was achieved, there might be a lot of bullet shell conducting and heating up before the current transfers to the sleeve. When terminating cable into a bullet, I preferred a nice blob of solder in the nose, if only to make sure that the wire would stay connected. Then there is the surface to surface connection between bullet and sleeve. Any lack of either area or tension between the two will generate local heat.