Transformers will change voltage, but they can't change frequency.
That's the first place it gets complicated. You also need to consider all AC appliances and equipment like HVAC, pumps, chargers, inverters/chargers, lighting, stoves, microwaves, washers, dryers, dishwashers, trash compactors, air compressors, isolation transformer, etc. The bigger the boat, the more of this stuff it will have. For each of these things you need to sort out of it can operate (or be converted) to work on the new voltage and frequency.
Then there is any generator. Can it be converted to the desired output? What about inverters? Some can be reprogrammed, but most cannot, especially with respect to frequency.
Then there is the wiring. Because of the lower voltage, 120V wiring it generally heavier than 230V wiring. That's good, assuming you are converting from 120V to 230V. If you are going in the other direction it's more problematic. Some devices could be converted to 240V, but not all.
And the wiring becomes even more complicated if there is a 120/240V split phase system involved. With a 230V single phase (euro) system, one conductor is neutral and is grounded, and the other conductor is the 230V line. In a 120/240V split phase system the two conductors that make up 240V are both ungrounded and 120V relative to ground. If you just plug a 230V boat into a 240V North American outlet, the whole grounding system will be live to the tune of 120V. Not good.
The best I can suggest is to NOT take any blanket advice as anything more than a example data point. The only way to really understand what you are getting yourself into is to do a detailed inventory of the boat's electrics and come up with a conversion plan. Boats vary hugely, and there is huge variation in how completely you can convert a boat. Some people keep the boat at it's native voltage and frequency and just convert where shore power enters the boat. Others want a more complete conversion with local outlets, and appliances built for local power so they can be sourced and replaced locally. It's also very different if you are taking your boat somewhere for a finite period of time, vs moviong it permanently to a new power system.