Its usually not practical or economical to rewire the entire boat, especially for older boats. The wires used for 230 volt are going to be smaller than the wires needed for 120 volt and so to change the boat completely would mean changing everything including all the wires. This is more work that it seems as even on simple electrical systems the wiring can be quite extensive and it was built in to the boat it may be hard to access or replace.
For boats like this the best option is usually to get a transformer. A suitable transformer for 16 Amp service is relatively small and inexpensive. Together with a new main breaker, a new shore power cord and main inlet, the whole project can be done for somewhere around $1350. The parts list below shows what is required.
Starting at the dock end you will need a new shore power cord for 120 volt 30 amps and you will need a new Deck Inlet on board the boat to connect it to.
The transformer only changes the voltage in this project, it wont change the frequency. If you have pieces of equipment on the boat specifically designed for 50 Hz they may not work exactly the same, for example the clock in your microwave may run faster on 60 Hz than on 50 Hz. Normally this isnt more than a minor nuisance but occasionally you may have trouble with electric motors overheating. This whole project is for European 16 Amp service only. If your boat has 32 amp service then this project gets exponentially more expensive, the transformers get heavier and bulkier and of course the costs go up.
Next you want to have a main breaker inside the boat. To meet the ABYC standards this needs to be a 30 Amp ELCI breaker located within 10 feet of where the power comes on board. The Blue Sea 3116 Surface Mount Breaker Panel shown here meets the standard. The ELCI protection offered by this breaker gives you a 30 mA trip and is primarily to protect swimmers in the water from electrical leakage. You will probably find there is something similar as part of the original installation but as it will be a 16 Amp breaker you won't be able to use it.
The transformer shown above is what changes the voltage from 230 volts to 120. It is rated for 32 Amps and so is appropriate for a US 30 Amp service. As well as changing the voltage it also gives the boat electrical isolation from the shore which is useful for preventing galvanic corrosion. This is a relatively compact device that needs to be mounted in a dry area. Its approximate dimensions are 14.25" high x 10.15" wide x 8.5" deep and it weights 53 Lbs.
For 30 Amp service the wire before the transformer needs to be Triplex 10/3 awg, after the transformer you can use the original wire which will be less. If a circuit breaker was located beside the deck inlet then you can take the output of the transformer right there, otherwise direct to the main breaker panel.
If you are taking your US built boat the Europe you can do this project in reverse. The concept is the same and the transformer works both ways.