SD - Are you talking about single phase or 3-phase AC? I think the power generation companies all make three phase power, which factories and large stink-pots use direct - so three wires, one for each phase, a neutral wire and an earth wire makes five. Most households receive just one phase, so the phase wire, the neutral and the earth. But there are odd countries, like Germany, that use the neutral wire as the earth. I don't think they are supposed to, but remove any socket off the wall there, and you can bet the earth pins are wired to neutral and there's no earth wire in sight. (This is to drive the Brits nuts, who are earth wire addicts, it's very effective too.)
Omatako - you could keep your boat on 110 VAC by installing an isolation transformer. The one I have transforms between 220 VAC and two 110 VAC (or in reverse). Then when you plug into 220V shore power, all your applicances will see 110 V.