Sailing south you won't need the heaters - problem solved :-)
I'd guess the 110v feed is an add-on by the former US owner of Kristy, not original Amel, correct? What wattage are you drawing on your heaters - many are in the 1800w range. Sounds like you've got more than one - two would present 3600w which at 120v gives you the 30A limit of your cable - no wonder it's hot!
If you need the heaters until you go south, can you run a separate additional 110 line out to the dock for one of the heaters? ... or get a 220v heater?
Seems you really needn't rip into the Amel wiring at all.