Firstly a complete re wire is usually not neccessary though it is often advised that it is. Most 220vac cabling will be over speced, so for the outlet circuits simply leave the existing the breakers , accept its will be at half max 110vac power, which is rarely an issue) and change the outlets ( ie less then hundred dollars). Then that typically leaves the fridge, battery charger and immersion. The fridge and the battery charger might be multivoltage if your lucky. In this case it's unlikely the 110vac for either will exceed the 220vac wiring
so that can be left in place , but the breakers may need changing. ( though probably not). The immersion element will need to be changed to 110vac and probably a new cable pulled.
Again if you know what you are doing its not expensive. It's gets obviously more complex if you have generators and/or invertors etc
European mains boat wiring is typically 2.5mm2 which is equivalent to AWG 14 ( obviously you need to check) hence the wiring would be could be good for about 1500 watts and maybe 2000 watts over short ( upto 10 ' ) runs.
The shore power inlet cable from the inlet to the panel may need updating , examine its rating
So if it's a fairly straightforward system it shouldn't cost 1000s if you know what you are doing.