That is a very complicated question. There are many questions to be asked before one could answer that. But, to start with the shore power cord and boat connector should be changed to bring it into code. Some of the wiring if not a lot of the wire will need to be changed or rerouted to eliminate the extra power
line. Fuse panels will need to be changed as well as some out lets removed. And what utilities ran off of the 220. what are you going to do with those? One would need a lot of info about the vessel to deal with this one. It will a little costly too!