Well, amazingly enough Dangerous Prototypes had a design up just this morning to do exactly what you're looking for:
<EDIT: Never mind, that's DC only. The circuit design also only supports 7A, but 20A would be possible with minor changes>
I'd go with an electric water heater element. A 2.0KW/120V element would run about 17 Amps, and it'll be pretty rugged and pretty cheap. If you leave it bolted into a hot water tank, you could run it for hours and heat water for the sinks in the bathroom at the same time. If not, put it in a steel pipe to protect it with a fan mounted in one end to exhaust the heat, and don't run it for more than a few seconds at a time...