The standard voltage on all yachts is 12V. In order to use electronic equipment that is 220V/120V you will need an inverter suitable for use in a cigarette lighter. You may rent an inverter through your charter company. In order to use the 120V outlets on the yacht you need to be connected to shore power.
Generally there are plugs in the saloon, at the navigation table, and in the kitchen. Depending on the yacht, plugs will also be available in the cabins.