An outfit called Powerair is releasing a generator that runs on zinc pellets. It produces no dangerous exhaust (thus can be used indoors). The zinc can be reprocessed out later for recovery and reuse via electrolysis. Sounds like it could be an interesting thing on a boat. Check the gallery for pictures.
Powerair representatives told us commercial models will be available in about two to three years for about \$1000 a kilowatt, but that price will eventually come down to \$600.